Thanks Thingybobby and Chuffa, I was beginning to think that nobody had the answer to my problem!

I have renewed enthusiasm for this project now and will get on the look out for some suitable wood to build them and will take your advice on filling from other areas on the allotment. I have inherited 3 compost heaps, so will hopefully have lots of other matter to top them up with too. Great pictures Chuffa, I am impressed with your handiwork!

I had a bit of a hare-brained plan to fix strips of copper to the top edge of the wood to stop any slugs and snails venturing over and eating my green stuff, but was dismayed to learn (on this site) that the little critters come up through the soil... would this therefore be a complete waste of time or help a little with keeping the numbers down? Maybe some beer traps (something else I learnt to do here!) in the raised beds would sweetly kill off those that defeated the copper?!