Cabbage Root Maggot

Recently lost quite a number of my cabbages and cauliflower to what I suspect is cabbage root fly, roots have maggots through them, even though I had brassica collars on. Infact nearly all of my fellow allotment holders are in the same boat. Is it possible that our manure could already have had the larvae in it. Is it ok to replant new transplants in the same area. A friend had suggested using micromesh, and I have been trying to source it in the colour green as I was hoping to make a semi-permanent cover to match the colour of my raised beds. Fussy I know but the plot is a blank canvas for my art lol, trying to make it ornate at the same time as grow our own produce.

You seem to have such a big problem with brassica root fly that I would recommend you try 'Nemasys Grow Your Own'.

It's a biological control containing nematodes and is active against: carrot root fly, cabbage root fly, leatherjackets, cutworms, onion fly, ants, sciarid fly, caterpillars, gooseberry sawfly, thrips, and codling moth.

http://www.nematodesdirect.co.uk/7-nemasys-grow-your-own

Google around for the best price. 

You will need to net them against butterflies but that netting is much cheaper than the very fine netting you would need to keep the flies out.