Clematisa, there is no one 'best time' to sow vegetable seeds - in fact you can sow different things during most months of the year. Toanswere your question satisfactorily would take a book and I would suggest that you buy one. You could also send for a seed catalogue from one of the merchants. They desribe the seeds , eg strongly flavoured onions, and adviser on when to sow the. The catalogue that we use can be found here
http://www.organiccatalogue.com/catalog/
and I would suggest that you send for a paper copy that you can keep as reference all year.

You could also check out my blog on growing by the moon... Bulba has a nasty bug so apologies for his dire spelling but his advice is correct! Right now is not really a good time to sow any veg. seeds. If this is going to be another long, cold winter then sure you can start them in a propagator but then what do you do when it is time to pot them on? Keeping them indoors is not a good idea as they get drawn. Now is the time to drool over seed catalogues and decide what you want to grow...

Thanks both - I got Carol Klein's book for Christmas a few years ago - so I'll see if I can find it and have a look and then I'll check if I've any veg catalogues in my mail - I usually get them from lots of places but not sure if I've kept any recently, if not I'll have some fun sending for them!