We had ours done late last year. Replaced an old floor stander in the kitchen with a new Worcester/Bosch Greenstar Ri in the upstairs airing cupboard. Our’s is a three bed semi with a single storey extension with another bedroom. Over the winter period the house was the warmest it’s been in the 8 years we’ve been here without the boiler being at max output and the thermostat on 20C.
Managed to salvage a few bits from the old system, timer, circulating pump, thermostat but needed a changeover valve.
With rerouting of pipework, flushing the system and the fitment of a muck-filter system came to just under £4K. Work included the electrical work as well.
Boiler was installed as hot water and heating and we kept the hot water tank as I’ve heard too many stories about combi systems breaking down and with three women in the house the thought of no hot water for their showers/hair washing etc was not a pleasant thought.
I got some very good advice from someone on the old forum who is a heating engineer but I can’t remeber his name.