My problem is that the smallest combi boiler I can find is the eco
Hometec
EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The
companies literature rates it at 16kw ish modulating down to 2.5 or
3.1kw.
Clearly much too big for my house.

Not really. If you replace it and don't go the combi route, you will
need to replace the hot water storage cylinder as well with a fast
recoivery one to comply with the building regs. That will swallow the
20kW output nicely and give fast recovery times.

I wasn't aware that the cylinder needed to be changed, only that if it was
changed, it would have to be Part L1 compliant. Any existing cylinder kept
would need to be fully pumped, with a thermostat added, though. Obviously,
an ancient scaled up cylinder with only a crappy add on jacket is probably
worth replacing anyway.

Christian.