O.K. - first thing discovered (or rather confirmed) as the major rewire progresses.
Always label up the CU comprehensively. Never trust the labels on the CU. Always test ALL the sockets on a ring (or lights on a circuit) the first time you power it down. Still don't trust anything or anyone (including yourself).
A couple of things have emerged: (1) The central heating does not have its own breaker in the CU - it is powered off the downstairs lighting. This is not recorded on the CU or by the CH programmer. No lights, no heating. It would have been nice to know beforehand. Normally I would have expected the CH to have been on the upstairs 13 amp as the main works are (were) in the airing cupboard upstairs. (2) There is one downstairs double socket which is not on the 13 amp ring - instead it is a spur directly off the CU (which is nearby). Unfortunately it is spurred off the upstairs 13 amp ring. So if you power off the downstairs ring you still have one live double socket. This is not recorded on the CU. Presumably done when the kitchen was extended and all the electrics were shifted around so that there could be downstairs power whilst the downstairs circuits were being comprehensively modified.
Second thing - the wiring under the kitchen floor is 'interesting'. There are more spurs than in the average Western and shares in junction box suppliers must have soared when the work was undertaken. I assume that the original wiring in the original kitchen was modified to include the new kitchen in the extension by breaking into the 13 amp ring and extending it - likewise the cooker circuit - plus maintaining the ring in the adjacent dining room and all without doing more than lift a few floor boards. Still, there were an awful lot of junction boxes - probably would have been better to break into as few places as possible. Always difficult when the wiring is embedded in plaster and you don't want to have to replaster the whole room.
I have been guilty of spurs and junction boxes in the past. You don't really appreciate what the result is until you remove the whole floor and some internal walls and look at the wiring in all its exposed glory. As I understand it this layout would not pass current wiring regs, but unless a fault had developed in a junction box then normal testing would not have revealed how many junction boxes there were.
Cheers
Dave R