I understand how it works in principle, but remain slightly sceptical

I've never used one. However, my understanding is that they do work.

What's the difference between twin and single impellor pumps? Or
more to the point, what's the reason for either?

A twin impellor pump pressurises both hot and cold gravity supplies. A
single impellor pump can either pump a single hot water supply (as you are
using mains cold, which needs no pumping) or can pump the outlet of a flush
mixer valve before it gets to the hose/head.

There's a lot of loss through the pipes in our house (1935 vintage) -
some touch floorboards and heat the floor!

Provided you have a cylinder thermostat with a large hysteresis, the primary
circuit will not be hot for long periods of the day. You will easily still
save money over an electric shower in energy efficiency. Also, it shouldn't
be difficult to lift a couple of floorboards and stick a thin layer of
insulation on the pipes if the heat bothers you.

Leaving it running all day I think would be wasteful as I'm only
awake/there for about 7 hours a day!

Then time it to run from 1 hour before you are there/awake until 1 hour
after. You should never run out then.

Christian.