RAM arrived from Bulgaria (Finally) NOS Fujitsu MB8264-15. It's been sitting there for a few days to be honest, but I was trying to get the Spectrum refurb done, but the keyboard was giving me some grief (its in a Lo>>Profile Keyboard case, and the tails had been badly repaired resulting in some solder tabs being destroyed, and when reassembling it, I struggled to get even pressure and the odd key didn't work).
I pulled the dodgy Chinese ram out and continuity tested all the address lines between the ULA and the Multiplexers, from the multiplexers to the ram, the address lines between the ULA and the CPU, the RAM and the CPU and the RAM and the VIA. all were ok
I then individually tested the new chips using DRAMArduino and fitted them. I booted using Mike's diagnostic ROM. on first startup I get nice straight vertical black and white bars. All tests up to the printer one passed. I stopped there because I haven't yet made the loopback connectors so that's the next thing.
Swapped the original ROM back in and started up fine, and "ping" and "explode" work! happy days.
So I now have a bunch of spares available (ULA, ROM, CPU, VIA, AY, and more RAM thats due to arrive). Unfortunately I have no way of testing the spare AY and I'm not desoldering the one from the Oric if I can help it. I am able to test the rest (CPU and VIA in the Beeb)
The original RAM was clearly bad from the beginning, evidenced by the dodgy $ being displayed randomly around the screen and the available memory only being 315 bytes free. Me shorting the regulator cured its headache for good, but how I never killed anything else (apart from the multiplexers, possibly? I'll re-test the originals), I have no idea!!
Thank you all so much for your support. Couldn't have done it without you.