Andrew wrote:

There are still Baxi Bermuda gas back boilers with the radiant fire
fixed to the front. One neighbour close by still uses his. That
has 28mm gravity feed to the hot tank and 22mm pumped rads,
with 4 tapped cores in the side of the Hx. House was built 1976

The houses on this estate were built in 1973 with gas backboilers, mine
was replaced by a cast-iron boiler before I bought it. Several
neighbours have fitted a condensing combi, and had to replace tem
already, my lump soldiers on ...

approx 11,000 kWh/year of gas, which covers heating and hot water of a 3
bed semi, at 2.7p/kWh that's £311/year ... where's the sense in
replacing it?