OK, the rads are all hot, but are the instructions correct in
recommending this high setting? Is this a feature of condensing
boilers? Would I waste gas by setting back to a low output?

Run it as low as possible, whilst still being able to adequetely heat the
radiators, and being no less than 5C above the setting on the hot water
cylinder thermostat. (i.e. if the HWC is at 60C, run at 65C, unless this
makes the radiators too cold).

If you have the built in diverter valve, you can set different temperatures
for hot water and radiators. In this case, set the hot water 10C above the
cylinder thermostat setting (for rapid recovery) and set the radiator temp
as low as possible, provided the house still warms up in a reasonable time.
In extra cold weather, you can bump it up temporarily.

Christian.