Winter growing tomatoes ?

In my experience you have to have very high light levels to successfully grow tomatoes in the winter as well as lots of heat. I've tried special daylight lights with plants sown in January but I found it did little to avoid leggy plants.
As it's not yet July, any sown now would grow rapidly and fruit by the autumn anyway. You should be able to keep some plants with fruit ripening up to Christmas anyway if you have heat in your greenhouse. Usually what happens is the plants die back with shortening light levels and higher humidity leads to leaves and stems rotting.
The best thing to do is find somewhere to hang up whole trusses of green tomatoes with parts of the plant to ripen slowly in dry conditions during the early winter. The other thing which is advised (and I tried once with some success) is to wrap each green tomato in tissue paper and put them in a box to ripen slowly indoors.
The simplest and best way to have 'fresh' (tasting) tomatoes is to bottle them in kilner type jars. I did this for the first time last autumn and we had fabulous 'canned' tomatoes through a lot of the winter.

Thank you very much for your advice,I shall do as you say,get more started now with a view to having some to ripen later.
My greenhouse is a bit crowded with peppers/chillies and aubergine,which left less room for the toms.

I do have some growing outdoors but the blight last year has made me a bit wary this year,and I dare not count on having a good crop ! hence the plan to grow a later crop inside