This is probably the best guide to keeping hens on the internet http://keeping-chickens.me.uk/

Assuming you are allowed to keep hens on your allotment you or someone is going to have to visit them morning and evening to let them out of the hen house and shut them back in at night. In an urban situation foxes are likely to be a problem. You'll also need to make sure they can't fly which means clipping one wing. To be honest I wouldn't keep hens unless you have space in your own garden... Bulba and I kept for several years and you do need to give them care and attention.

On an allotment you will need to be very sure they can't get onto your neighbours patches as hens scratching and digging up worms and grubs makes one heck of a mess and they'll eat anything they can get their beaks on.

They are already down on the allotment as one of the elderly tenants cannot keep them anymore due to health reasons. Thanks for the advice so far

Ok, had a year with chickens until mr fox visited one night. The corn and other food that chickens eat will attract rats, there's no doubt about that but i don't think they pose too much of a threat to the birds...they may eat eggs if hungry enough but that's about it.

Best thing to do is to make the run as rat and fox proof as possible. Dig the wire into the ground and have it double thickness if you can. Putting the run on slabs....with an area in the middle for the chooks to scratch around in is a usefull idea but i can't emphasise enough how important it is to make the run as fox and rodent free as possible.

You'll love the chooks, my girls were fab and great characters and i do miss them.