"However, any installer specifying a 24kW combi sounds like the sort
of cheapskate that wouldn't even know what a boiler interlock was. "

Arent you making an assumption on the size of the house the boiler is
fitted to?

Nope. It is a combi, which is sized for water demand, not the size of the
house. 24kW is only just OK for a shower.

Given the marginal price difference between a 24kW and a 28kW, it seems
pointless saving yourself two pounds fifty and losing the extra power, which
makes all the difference in such a marginal installation.

Christian.