ChrisR wrote:
There is a rough cut/paste key by Steve Falk floating around somewhere ... not sure whether I PDF'd it or not (I can email it if I have got a copy). The same goes for Rhinophoridae & Sarcophagidae ... but they are all a bit old and new species may have been added to the British list since then. But, saying that, I use them here :)

You have to really pick a family and then ask whether there are keys for them ... each family is different. Some groups are only really accessible by buying quite expensive keys but the trick is to go for a group (like Tachinidae) where there are free keys available ;)