winter brassica problem

Possibly cabbage root fly? 
http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=646

Hi monkeygee,
 
I've had a similar problem with a couple of our cabbages keeling over, on inspection there were  5 or 6 Wireworms in the soil of the cabbage base/root area. I promptly fed these to my hens !!
 
The worms are brown/orange in colour and 10-20 mm in length x 2mm in diameter.

A neighbouring plot holder suggested that to try to eradicate any others in the soil,I bury a piece of cut potatoe just below the surface ( marking where they are placed ) and remove and destroy after a week or so.

have now identified the problem as cabbage root fly , how can i remove them from the soil
, i have cleared the brassicas and am planting elsewhere