Dave Liquorice wrote:
[snip, thanks for useful comments]

The only reason for adding the hot water to the stove is that the
kitchen in particular is a *long* way from our existing hot water
cylinder and it takes a huge amount of waste water ...

Are you on a water meter?

Yes, but in addition think about all that wasted heated water.

It seems to be about a sink full of water before it runs hot, that's
quite a lot of wasted heat as much of it will be lost to places where
we don't need the heat (under floorboards etc.).

For some reason the sink in the kitchen takes *much* longer than
anywhere else in the house for the water to run hot. It is a fair way
from the hot water cylinder and I guess the pipes must just take the
long way round to get there!

--
Chris Green