Sowing onions - help please.

I've always sown mine in December or January,but I think if you are growing for showing then they seem to start earlier. Not that I really know,I've only grown from seed a few times.  Unless you've got sets and they are sprouting now then you might get a crop.
Or if they are over wintering sets then I usually put mine in about October.

Is this where you read all about it?
http://www.realseeds.co.uk/onionsoverwinter.html

I agree with the previous comments. August is early to sow.
Sturon are readily available as sets in the new year, so if your seed sowing fails you will have a backup plan.

So experiment, sow some in August/December/Spring. Sow in pots and in the open. Keep notes and decide for yourself.
I don't grow exhibition onions but I do try to follow these guidelines.
http://www.johnsons-trade.com/uploads/resources/20_wwj11-kelsae-flyer-v2.pdf

Producing a good crop of onions is very rewarding.

http://www.realseeds.co.uk/onions.html  gives a good explanation of what to do. 

The nub of the advice is " For full-size onions:   Sow it around Aug 18th in a closely-spaced row. At the start of October they will be a decent size and you just transplant them out to their final position spaced about 15cm apart. As soon as the weather improves in spring they'll be off and away, giving you earlier / larger onions than you would get otherwise. " 

They recommend Paris Early White or Sturon if you have mildish winters.

I've never tried winter onions from seed before, I'm always still thinking summer not winter in August. I might try now you have given me a heads up.

I grow that one Auntie. A lovely white one and you eat them fresh. WEll I did 

Thanks for that AS.