Having isolated put a stop end in the hot water pipe out of the
boiler, so that only a single (washing machine) outlet is in the run,
the hot water problem remains.

The tap needs to be turned fully on to get the boiler to fire up. This
is a much greater flow than used to be necessary for hot water, and
the kitchen sink and shower are unable to trigger it.

I am therefore sure the problem is within the boiler itself. All other
problems have been fixed. Bosch will be back in a week to take another
look, but in the meantime does anyone have any ideas on where the
problem might lie, eg the hot water diverter valve / flow switch
sensor thingy?

Thanks,
Richard