This'll be my first year. From what i understand, get them in from now to xmas, they need heat to germinate and then you can take them off the heat. Prick them into pots when they're at the "hook" stage. Artificial lighting can help growth a lot.
Mine will probably go in this weekend.

I used to grow them Steveg I was always told to sow them Boxing Day by some older gardeners who used to show at the same place as I did. Obviously they need heat to germinate which I did indoors and then put them in a cold frame about beginning March then plant them out when the soil was suitable to work down.Not sure why but I was told to water them in dry weather with a sugary solution , and I did win 1 or 2 prizes, so it worked.

I was told, again by an older grower, that he waters his onions with diluted molasses. Apparently it does make the onions sweeter.

According to our friends who grew onions they get planted on the shortest day of the year (Boxing Day) and harvested on the longest day.