Surely you're telling porkies there, Tony
I really don't understand if and how a combi can replicate the vented system here. We've a power shower (though don't use the 'power' bit) which takes the hot and cold water above the bathroom and comes down again. I don't understand your 'pressure pump bit. There's no way we would want to install an electric shower with its 10mm2 radial to the c.u.
I'm learning from this thread but still can't see how a combi would be as efficient. Okay for loss of cylinder and tank removal benefits but would I need all the paraphernalia associated with updating my vented system (apart from not needing a new cylinder)?
In the eighties and nineties, combis were bad news for those I knew who had them. I'm sure things have changed but I've had new indirect boilers replaced before, without all this complex digital stuff. Guess reg's may have had sth to do with that.