When I was cranking the engine with a bar and wrench there was defiantly resistance as I cranked it. There was defiantly no grinding sound or locking as I turned it. I have not tried the compression test yet or tried to find TDC then check cams. I will try this next.
I might be barking up the wrong tree however as the engine did start and sounded smooth for a second then it cut out and it keeps coughing/stuttering thereafter.
Exactly how my mates car was, sounded great for a few seconds then nothing. Good luck.
Terry 
this is a way to find TDC without the use of the crank timing mark and i way i’ve been doing it for 20 years on engines of all makes and models line up or close to it your top timing marks remove the plug from no1 and turn engine by hand while looking down through the plug hole you will see the crown of the piston then using a chop stick length of threaded bar place it into the hole and against the top of the piston while applying a small amount of pressure with the tip of your finger turn the engine slowly you should feel the piston coming up then it will stop for a split second then start to move down again at this point wind it back slightly till it comes to a stop again and you have TDC then you can look at the cam marks and see how they sit hope this helps
mark
Top tip using a depth gauge , only thing i would do different is a full revolution again , winding it back can cause the belt to slacken off against the tensioner and slip .
Craig 
I must say what great advice has been offered to Peter so far, a pleasure to read. I’ve done 3 belts on these now, and the info imparted has echoed everything I have come across when doing this job. You are a number of notches out on your cams, and these need to be spot on - the punchmarks on the cam sprockets should be exactly opposite the small diamond castings on the cytlinder head. But, you must set your No.1 cylinder to TDC first.
You will find that when you rotate the engine to come round to TDC, your cam timing marks might well be all over the place. Rotate one more time and trhen take another look at the cam marks. This time they’ll be almost at their points. But from what I’m reading so far, your timing is out, although not by yards by any means. As Marty said, only rely on the crankshaft sprocket timing mark to observe TDC at. To get to the crank sprocket, you’ll have to loosen the ancillary drive belts and then remove them from the crank pulley. Once you have plain sight of the crank pulley, you can insert a large socket onto the crank pulley bolt and undo it. Then it’s a case of wiggling the crank pulley off of the crankshaft. The bolt’s a tight one, and will take an effort to loosen, and you’ll need to prevent the engine/crank from turning while you do it. Then, once you have clear sight of the crank sprocket, you’ll see the timing punchmark on it. Ignore the locating dowel nearby - that’s used to assist positioning the crank pulley when reassembling.
Although I’ve never performed an “in situ” retiming procedure, I believe it is quite do-able, but you will need to get access to the cambelt tensioning eccentric pulley to carry out this work. There will need to be a temporary setting in place of the hydraulic tension device whilst you slacken off the eccentric cambelt idler. Then you can reset your cam sprocket timing marks before resetting the belt tension.
Marty advises that you’re best bet would be to get your car to Eurospec in Guildford, and I second that. However, if you are up for a challenge (not impossible to do by any means), then this will put a huge smile on your face when you’ve done it.
Happy to help or advise if you think you’d like to have a bash yourself.
Good luck with it, be as patient as you can - it’ll work out fine.
Mike
I usually use a long 1/4" extension bar, but the principle is the same.
Marty
yeah i no what you mean marty anything that you have available really thats solid and fits in the hole i have seen it done on sailsbury plain with a twig from a branch found nearby but i wouldn’t recommend trying it lol
Oooerr!!!
Terry
I use a piece of weld wire but same principle. The pulley that fits but puts it 90deg out is from the 6G74 3.5l engine.
Once you have found TDC make your own timing mark so you don’t have the problem in future. And as Craig said don’t wind the engine backwards!
Having done a compression test on the front 3 cylinders with throttle wide open and ECI fuse out I am only getting 80psi to 90psi on all 3. Not done the back three however I would imagine that they would give a similar reading.
A normal reading for this car should be 135 lowest to 185 highest. I am a bit under the lowest reading. Would incorrect timing cause this low compression?
How many times did you crank it over ? need to do it more than once or twice ,
yes incorrect timing would cause low readings , especially if valves are bent , surprised to see the front three all have the same low reading though
Craig
10 seconds I was cranking so I would imagine about 5 cranks.
That should have been enough to build up pressure i would have thought , so i would say at those readings you may as well check the rear bank , you are going to need to properly check the timing though , is it a decent compression gauge you have ?
Craig
I got it from Screwfix so not an eBay job. Going to take the rear intake manifold off for the 3rd time 
Probably take me 10 minutes now to access rear banks because I have done it so many times lol.
thats the spirit 
once you have done it , it is quite an easy job isnt it
Craig
Yup at this rate if I get the car going I will be the GTO guru. Its started raining now so I am going to leave it until it stops, which will probably be tomorrow now 
The only other thing I am concerned about is that even if compression was 90psi max, then this should still turn over. The fact that when I first tried cranking today it turned over for a second then stopped would suggest this.
Also, I can’t get the engine warmed up to correctly test the compression as it won’t start in the first place Haha. Ah well, I might send ECU to the doctor to get it tested in the meantime. When I tested the ECU with a analogue voltmeter it was a constant back and forth which would suggest no problems.
Just to check did you have the throttle wide open when you were doing the compression test? Foot should be flat to the floor?
Well ECU came back as fine. Checked TDC mind you on cylinder one and 1st cam is forward by 1 tooth and the others by 2.
Tried the card trick to adjust cams which worked well on the first cam, however when I did the next cam, I didn’t realise that the belt was slackend from doing the first cam. The second cam rotated on its own as I was cranking to get the cards out. I tried to correct it by counting teeth from the markings and held a metallic/clunk when i rotated cam with a spanner and I was unable to adjust the cam any further.
Going to use bulldog clips when I do the other cams. Hopefully I havn’t caused any damage and I can wind back the cam. Does the belt tighten up as you crank via the crank pulley?
As long as you turn it in the right direction!!
Right chaps,
Thank you for all your help. After adjusting the timing via TDC (which was a lot easier than trying to see the mark on crankshaft pulley) she is now running again
Now I need to find out why it isn’t idling 
she runs alright if I keep my foot slightly on the throttle. Also I suspect the gasket between the exhaust manifold and center section has gone as it looks foamy and exhaust smoke is escaping. At least I have the engine running for now.