My plot is completely overun with woodlice, they took all the roots off my onions, ruined tomatoes and every time I lift a bucket or a pot there are hundreds of them.
Anybody got any suggestions on how I can at least reduce the numbers.
Get chickens!
In all fairness to the woodlice, they're not normally considered to be a pest. They like decaying vegetation & will shelter in the holes but are very unlikely to be the culprit as far as your onion roots or toms go.
Borrow some chickens as ollie says... they love 'em... ;D
I would be inclined to look elsewhere for your problems. Woodlice will nibble at seedlings but I've never noticed any trouble on healthy growing plants. I think they are a vital part of the allotment ecology spending most of their time hoovering up vegetable detritus.