If it faces West then you probably won't need any heating whatsoever. At most, a small oil-filled radiator with thermostat control could be used as a back-up. We use this at this time of year when our North-West facing conservatory misses out on a lot of sunlight. Enjoy.
It rather depends on the quality of the insulation in the translucent roof panels, as this is where most of the heat loss occurs. Your options apart from the above are electric fan heaters, electric underfloor 'blanket' in the floor (use about 200W per square metre), extending the rads from the CH (most complex, but possibly cheapest to run).
We just use a 2kW convector (about �25 from Argos) in ours with no problems at all, though we do leave the doors to the dining room open which lets some heat from the house into it. Ours is 3.5m x 5m and North facing.