All is well!
Finally, some good luck. All is well with the engine. The engine builders have pulled it down and everything looks to be perfect, so thank fark for that!
Turns out the GTI-R oil pump had stress fractures on the backing plate, as well as slightly worn gears. So as a precaution I'm just going to buy a new pump assembly and have it modified to go on once again. The cost of doing ths annoys me a bit, but it's better to be safe than sorry nd a lesson learnt: Don't use any second hand parts in a fresh engine build! It was the only second hand part, and lo and behold it was the one that failed... unbelievable.
Despite the extra cost involved I'm just super relieved that the engine is A-OK.
I've entered the car into Sandown for late June, so here's hoping I can finally get in the thing and drive it.