So, have had my logic board re-capped by Phreakout (cheers again!), and all is going very well indeed.
Until today. Switched on my SE/30, and the chimes of doom spring out with a simasi mac pattern on the screen. First thing is take the back off and have a look around. Nothing burnt / melted / on-fire. Reseated memory and ROM. Reseated floppy and hard disk cables. Partly re-assemble (without case). Test. All okay. Put case on. Test. All okay.
Then I stand the Mac up on the desk, switch it on, and get the chimes of doom back. Simasi mac as well.
Repeat. Same thing.
And again. Same thing.
Until I leave Mac at simasi screen for a couple of minutes.
Restart once more, and we get a single chime, and a fully functional SE/30.
I'm missing the 'paper' thing that separates the logic board from the (looks like plated for RF shielding(?)) case internals. I've tried a piece of A4 pinter paper, but this doesn't seem to make any difference. I'm also working with some potentially iffy RAM - there's 68MB in the machine, but 64M of it is from a printer or something like that. This RAM has given me some chimes of doom before, but as I've said - everything was going so well...