It has to be remembered that what ever thermostat you fit it will have a cut
on /off range of around 5f. so no stat is going to be very accurate.
You could get a room stat and hang it behind the heater with a poly bag
fastened to the cable above it, and the bottom open just to keep any stray
water off it.
It is always hard to get an even heat distribution in any greenhouse, the
larger the harder unless you can duct the heat around.
Always try to have enough heating capacity in hand for the one night when
the temp. droops down to -10 or -12.
If it looks like being really cold remember that a few sheets of newspaper
laid on top of the plants will help to keep the frost off.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk
***2004 catalogue now available***