That's what I ended up doing, Mr. D. The seat lock, as expected, has a completely different barrel and cylinder, way too short when measured in situ, so no point even removing it to have a look. I cleaned everything, but it all came down to wear in the key path allowing the wafers to still stand proud. No choice but to get the buggers sanded down with a Dremel drum then dismantle it all again to clean all the metal filings out. Still plenty of meat on the wafers, it does what it's supposed to do, the key won't come out when the engine's running, all physical and electrical functions are as before. Graphited up, just redy for a ride out, now.
Ah, bollocks.