There is gas to the house but only runs to the downstairs cooker - much
as i thought it's going to have to be a chunky electric supply and
another large immersion type wall mounted heater fitted as the
replacement.

Use the gas. Running the pipe to a convenient location will probably be
cheaper than installing 3 phase. Then use a 28kW (or bigger) combi boiler.
Then, if you ever do want to install central heating, the boiler is sitting
there just itching to be hooked up.

Christian.