or (b) to fit a larger expansion vessel. Do you agree?

Yes, although there is no need to remove the old one. Just stick an
additional one on in a convenient location.

Does anyone know if a larger expansion vessel can be fitted to a
Britony 80 combi boiler, and if so, how much one costs, and is it an
easy job to fit it? I'm a bit woried by the manual which says the
expansion vessel is part of the boiler's chassis.

Leave the built in one alone. An additional expansion vessel will cost a few
tens of pounds and is easy to fit. Indeed, I would regard it as a
particularly suitable project for someone interested in learning a bit about
plumbing. It can be installed just about anywhere on the system, but
preferably not zoned off.

Christian.