Thanks for the replies. Regarding insulation and heat loss, I do plan
to insulate it well. It is in my own interest to do so. I will also be
putting a rad in there and using approved double glazed windows and
doors. Will this not suffice? Also, the house is a mid 20's semi with
solid walls, so the new bit will probably keep the heat in in better
than the rest of the house!! Whoever errected the conservatory in the
first place also layed the concrete slab right in front of two air
vents that vent the underfloorboard area of the rear of the house
thereby completly blocking them !!!!

Gazza