DRAM 128K system has stopped working

[CheshireNoir]

OK Folks, got a chance to do some more testing.
Now dropping in the ut212 test ROM, I'm still getting no video, same as with the default ROM. Hitting the reset button I get a really flickery version of the boot screen.
https://photos.app.goo.gl/VUnEd4nNjbxNiCqk6 <- Animated! 5MB
or
    <- this is flickering like the display draw is being interrupted. It's more visible inn 
I've done some probing around and I'm now suspicious of the reset circuit. (Thanks Ernest)
I've checked C43 out of circuit, and it's a little on the high side at 715nF, but is within ±20% that caps tend to be rated to.
On the scope on pin 18 of X5 (RESET) I see it only go high as far as I can tell instantaneously (Or at least with a VERY short period). I would have expected it to stay low for a measurable part of a second. 
Should I be seeing more activity on the NMI pin? It stays low unless I press the reset key. Then is goes high and back down again. Nothing else is generating NMI calls?

I have the schematics out and I've been chasing my tail around with this for a bit now. What should I be checking next? (If you can't tell, I'm not exactly a whiz at this level of electronics. I know enough not to hold the burney end of the soldering iron)

Thanks for putting up with my ramblings. Happy to test / desolder and test out of circuit / socket and replace whatever people can suggest.

Cheers!

Chesh