Its a v8 mate.After much searching I've found my leak. It's from the intake manifold right at the back. It doesn't happen till 15psi. I'm guessing there's a mismatch between the angles on the head/manifold .The rubberised gasket must take up some space but higher pressure leads to leaks. I did seal it with rtv but I had to do that bit twice for a reason I've forgot!
Check them angles when rebuilding with a feeler. The block was skimmed as well as the heads.

If the heads & block have been skimmed it will throw it out of alignment with the intake unless you have the inner faces of the heads skimmed to match. I'd also use some threadlock on the bolts as mine always seemed to back out & be loose after a while, especially the rear bolts.