chris French wrote:
snip
Seeing as the geothermal heat is 'free' (ok electricity is needed for
the heat pump), I can't see what the fuss if it consumes more energy
than needed. and if it pays itself back in 6 years seems good value to
me

But, it's not.

Let's be optimistic, and say that the heat pump takes 1Kwh to pump
7Kwh of heat. (about the theoretical maximum efficiency for pumping
from 0-40C.)
But, electricity is some 3-4 times dearer than gas.
So, unless you are comparing it with electric heating it's not looking
so rosy.

It's (eventually, assuming it lasts for ever) 7 times cheaper than
electricity.
But, it's only about 2 times cheaper than gas.

Payback can happen faster if energy prices rise (gas and electricity
will tend to rise together unless we go nuclear), but it will never
outperform adding insulation unless electricity is free.

I'll take a guess that the walls of the church are 75cm stone on average.
That's a R value of around .6, or a U value of 1.6W/m^2/K.
To halve the thermal loss you only need about 10-20mm extra insulation.