I'm not looking at permanently installed gen sets - and certainly not
automatic changeover etc. I'm considering a small portable generator
(maybe
Honda eu10i or eu20i) powering a 6-way extension lead into which I would
plug the relevant devices, having unplugged them from the mains. [The
central heating is currently hard-wird into a fused spur - but it would be
trivial to change that for a 13A plug/socket (with a 3A fuse, of course)].

One of the easiest ways to distribute your generator power is to "back feed"
it into a ring main.

First of all, once the power goes off, switch OFF the isolator in the CU,
then switch off all the breakers.

Make up a very dangerous lead with the generator connected to one end and a
3 pin plug on the other, then plug this into a socket on your ring main.
This will back feed the whole ring main, so ensure only the devices you want
to run (taking into account the power of the generator) are switched on.

If you are careful, you can then switch on the breaker of that ring main,
together with the breakers of any other circuits you want to "back feed".
(NOTE DO NOT TURN ON THE MAIN ISOLATOR).

The main advantage of doing it this way is that all the earthing is already
taken care of, and you don't have miles of cables running all over the
house.

You may have to be careful if you have a split load CU with and RCD, you may
not be able to "back feed" across the RCD into the non RCD protected side
(e.g. lights) without upsetting the RCD.

Oh and when the power comes back on, make sure you remove the generator
before turning the mains back on !

Cheers,

Paul.