Growing Onion Sets

I have had 2 years of growing autumn onion sets. The first year was not good so changed postition and this year they were slightly better, still not massive.
Just read on BBC gardening site that I should manure a couple of months before but as I want to plant them this weekend that isn't going to be possible. What else can I do to ensure my onions grow to the required size.

As a test I planted some in my daughters garden which the previous owners had used as a small veg plot and with little effort hers grew bigger then mine ....

My experience is almost identical to Broadhavens and I've been doing as DD says. I've had reasonable success at autumn planting but I wish I knew the answer to growing spring planted onions. I'm not expecting massive ones - anything over 2" diameter would be nice!

You'd better ask an onion expert! I'm fortunate in that my soil grows good onions, so I've not have to experiment with soil structure etc. to find out what does the trick.

My soil, whilst not sandy is quite free draining and stone free apart from a random pebble.