Now I've got a SCART cable I was able to test my Enterprise 64 and the Enterprise 64 motherboard that I own. Happily, the complete Enterprise 64 appears to work flawlessly. The motherboard looks to have a display fault, however (and since I've tested the complete Enterprise 64 I know it's not the cable or display).
When it starts I can see the coloured "Enterprise" text but there is an issue with horizontal stability. Not sure how to describe it but hopefully the attached video shows what I mean.
Does this look like a capacitor issue? Would be really helpful to know if this fault has been seen before.
The good news is that since it's got as far as displaying a recognisable logo the board is probably in pretty good shape despite having been left as a bare board in storage for many years.