Hi Michael and welcome to GoY, is your soil well drained? Onions don't like sitting in wet. Also when are you planting the sets?

in well drained soil.plant out in late april/may after growing them on from 3inch pots from feb

Never tried starting them in pots we just plant late March early April time. I'm not convinced that starting sets in pots and then planting in the ground works... Mind you checking your actual post you are saying you get onions the size of a tennis ball, that is a pretty good size to me. Golf ball I agree is a bit small - so when do you harvest?

usually harvest late sept,inmiddle of aug i bend over stems to let more growth go into bulb.

Have they bolted? If they have they won't grow bigger. I believe your onions are of a good size. Usually big onions grow from seed and not from onion sets. I used to grow huge onions from sets when I had an allotment but it was from a specific variety and not from the growing conditions as such.

I guess you are just going to have to accept that tennis ball size it is...

My climate is drastically different from yours, Michael, but here are a few thoughts from my general knowledge about onions.
First, when you moved, did you go farther north or south? Closer or farther away from the coast? A change in climate might be responsible.
Are you still growing the same variety? Different varieties may have immense differences in mature size, and different varieties do best in different areas.
Have you tried applying kelp or seaweed? The potash in this helps the bulbs to grow larger.
What is the pH of your soil? Onions prefer soil from 6.5 to 8.0. Very acid soil will stunt the plants, resulting in smaller bulbs.
Also note Moon_grower's original question about drainage.
As Moon_grower has pointed out to me, bending the tops over is necessary in the UK to speed up the maturation process--tunic development and closing the neck--but it won't actually increase the size of the bulb. In fact, it stops the growth of the bulb. You might want to double check to make certain that you aren't doing it too soon, but you will still probably want to do it.