It looks pretty grim, Keith, but this is what I found:

http://www.allotment.org.uk/vegetable/brassicas/cabbage-root-fly.php

Products containing spinosad are also worth a try, but I'm not certain whether or not that's available in the UK.

Nemasys applied in April is a biological control that may be useful for the overwintered grubs that are in the soil
Covering with fine mesh may help.
And of course, move your brasssicas to an area not previously used for them.

True! With all of the insecticides going off the market--usually for good reason--crop rotation is becoming critical.