So, the good news is that the chips on the board tested out ok, so it looks like I've got a loose connection or dry joint, so time to be spent tomorrow with a multimeter and the schematic, joy.
I'm happier now that I know it's something daft thats keeping the STF from working, its just a case of finding which bit.
Good news is that I have more working spare parts than I thought but did find some oddities. IMP shifter works ok in my Mega but not in the STF and the -38 DMA I have surprisingly doesn't work with anything.
Think I'll order some new sockets for the ROM and maybe GLUE as they've been in and out so many times.
Also, looks like the blitter I tried in the STF doesn't want to play in one of my Mega's and didn't want to risk breaking the other Mega's bitter to find out.
Hopefully Icky's order of a blister from Aliexpress will work out, so I can get one too
Fingers crossed for tomorrows fault finding session.
If it ain't broke, test it to Destruction.