The difference between condensing while heating an unvented cylinder
or heat bank is minimal.

I've just had a thought (just rambling, not thought through). Would it be a
good idea for the cylinder primary coil return pipe to go through a plate
exhanger with the incoming mains cold? This would pre-heat the incoming
mains a little and reduce the primary return line temperature quite
considerably, giving good condensing and something for the boiler to "push"
against. I can see a problem that you might unnecessarily heat a short
section of DHW piping when the cylinder is recovering but no DHW flowing,
but this could be dealt with by insulating the section to reduce losses and
this 82C heated section would be sorted out by the thermostatic output
valve, before it hit the taps, probably.

Opinions?

Christian.