It was a graphics card failure. Apparently NVIDIA made a load of bad graphics cards which failed in most PCs and only particular Macs i.e. this one! It's a known issue so the repair work and parts are all done for free good news really. Thanks for your help!
yep, that's why I gave you that link....
original 1yr warranty extended to 3yrs on that graphics processor
If you're lucky, maybe if you ask nicely enough, they will upgrade your Ram or HD for you troubles