The UK one *almost* worked for 20 minutes before the screen inverted and the characters became unrecognisable. Searching around found Tom/Scouter3d's heimcomputer.de site and here, and Tony/tjewell99 sent me the schematics which were great for hints.
With an oscilloscope everything LOOKED ok, no stuck address or data lines, CPU clocking nicely, ROMs active. The VFD showed correct characters. I spent a while looking for a dump of the character ROM since I suspected an issue there and in the end pulled one from a spare board and dumped that. Replacement didn't help so I traced the video signal back from the RCA connector and put my logic analyser on everything between that and the Character ROM.
The French machine was dead apart from a reverse screen even after swapping the PWRUP and RESET caps so I spent a while testing the components around the startup circuit, even swapping the CD40106 that ultimately asserts those signals. Still nothing so I pulled the major components and built the circuit on a breadboard, worked first time. The CD40106 I pulled from a spare board also worked first time. Turns out I'd missed some leaked electrolyte on the holes from the original startup caps so after a thorough cleaning I soldered everything back in and.....it worked.
PRINT FREE only showed 13K though so Colin's (RIP) service manual to the rescue with the POKE 20000,0:?PEEK(20000) command returned 1 and looking at that chip on the store board it was obvious it had been damaged also by leaking electrolyte. I pulled those components and gave it a thorough cleaning. Replaced the caps and fitted a new IC416. - bingo! The VFD doesn't work though, voltages on the converter board are low to say the least so that's obviously died. Still, the machine itself is happy.
I took many pictures doing this which are on my collection's FB page - http://facebook.com/binarydinosaurs but I'll upload them here if people would like.