".......... but some folk say temps a yard down are a constant 50 odd
degrees regardless of air temperature above .........."

I doubt this.
When we lived in Hastings we had a borehole for our water, and it came up at
a temp of aprox. 43f all year round.
I have been reading an interesting article about a project in Holland where
they are drawing water from the Aquifer and using it to cool Glasshouses
during the Summer months, this is then pumped back down, then in the winter
it is drawn up and used to supplement the heating, it is holding to a temp
of around 50f.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk