you could try the wireworm Nematodes Sticki, not sure how effective they are as i've never used them..

You could laso dig the ground over roughly and let the birds have a feed. Then keep doing this every few days/weekend until you are ready to start planting.

Wireworm nematodes have a good reputation, but never had to use myself ... yet! Whenever i come across them I squish them with delight, but this has not happened often enough for me to use the nematodes innoculation. This website came up a couple of years ago when I was concerned and still seems to be going, so may be useful to you ... http://www.harrodhorticultural.com/weblog/archives/2007/02/wireworm_contro.html

Many thanks, SL Plantaholic and Avkq47

My sister says she will give this a go!
She says another allotment holder suggested they don't like fertiliser, has anyone else heard this?

She has caught those she has seen!

Could be that the fertiliser 'salts', ie potassium, nitrogen etc, deter wire worms coming to the surface. This might send them underground - just where you don't want them!! Dig over regularly and squish any you find. You can also dig in slug/snail pellets, which will catch wire worms and slugs alike. I would go with the wire worm control previously mentioned, especially as it sounds like you have a bit of a problem with them. I have tried to remove them and feed them to the birds, but they burrow and move so quickly this was an idea to nowhere!!

many thanks Avkq, i shall pass this on to my sister.