The stat needs to have 3 connections - common, demand and satisfied. When
the tank temperature is below the stat setting, common is connected to
demand. When the tank gets up to temperature, the contacts change over and
common is then connected to satisfied. Without this connection, the CH
won't work after the HW gets hot.

When I said wire it like for like, I meant in terms of function. Forget
wire colours and contact labels - they may be different between the old
and new stat. Demand and Satisfied are sometimes labelled NO (normally
open) and NC (normally closed) or vice versa - so you'll need to work out
which is which.
--
Cheers,
Roger

Thanks, that's good. I can use my multimeter to work that out, I was worried
about whether it had anything other than just a switch in it, sounds like
it's pretty standard to just be a c/over switch..
cheers