From the same 1990 Survey recently mentioned.
Recent research by the Camborne School of Mines at their site at Rosemanowas Quarry in the Carnmenellis Granite has been directed towards solving the engineering problems with the extraction of the geothermal heat from the impervious granite by the enhancement of natural fractures in the rock and the injection of recovery of water heated by the hot rock reservoir. Resuts from the pilot stage of this work have been sufficiently encouraging for the UK Department of Energy to finance a full - scale trial with 6km-deep wells recovering water at more than 170 deg. C and generating approximately 5MW of electricity. If this stage of the programme is successful it is calculated that the entire south – west batholith could represent a major new energy source Equivalent to 6000 million tons of coal, or the entire UK national coal reserves. (My emphasis).

Didn’t happen of course, or at least not yet.

Have uploaded an article with the latest on geothermal.

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Sounds a better, and more reliable (and continuosly available) idea than ******* stupid windmills!

Geothermal kinda splits into 2 types: low and high enthalpy (and medium). High temp used to generate electricity and low temp used in heating and cooling among other things
http://www.southampton.gov.uk/environment/environmentandpollution/Geothermal/default.asp#0  to an existing scheme in Southampton

http://www.dti.gov.uk/energy/sources/renewables/renewables-explained/geothermal/page17506.html 
There are a few reports etc about geothermal potential in the UK

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There's also been some reports etc about use of mine water for geothermal energy (low temp)

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