3.5 x 3m conservatory what size electric heater will heat this room on
a winters night, it is one of those pentagon shaped conservatories.

Full height or dwarf wall?
Dwarf wall insulated?
Double glazed or single?
Roof glass or polycarbonate?
Polycarbonate roof double or triple wall?
Glass roof double glazed?

Is convection or oil filled the best, I dont really want a fan heater.

Convection quicker and more effective, especially with fan boost for rapid
warmup.
Oil filled safer.

Both equally inefficient with the world's resources, so use sparingly and
only use devices with thermostatic control. If you have gas CH, leave the
door open into the house, as this is more efficient.

Christian.