Thanks for all your advice, I haven’t quite worked out yet if it’s all made me wiser or more confused, but I am certainly grateful for it!
A further question, if I may: after reading some replies would I be correct in thinking that with a Combi the hot water flow will be slower? That would be a problem as we already have a fairly slow flow and presumably our shower (mixer tap on the bath) would become unusable.
The hot pressure might well be higher than via an open vented system, but that depends on how good your cold supply is - both pressure and the flow. A normal combi has to heat the water instantly, so it needs to have a much higher Kw rating than an heat only boiler. The hot water flow via the boiler, might need to be reduced, to achieve the desired output temperature, whereas you can draw hot water from a hot cylinder at whatever speed the system will allow.
I have an open vented system and visitors who have a combi at home, are often surprised at just how quickly the water runs hot and how quickly a bath can be filled.