Today I finally went to the AFCO and inquired about the asthma issue. The RAF representative basically said that any record of asthma in your history is a bar to joining as aircrew. However, he said that if I was able to get a letter from my GP or a specialist saying that I'd never needed an inhaler (which I'm confident of) it would pretty much be enough to get in (although Cranwell would have the final say). So the message is simple, a history of asthma is a problem although you will probably be ok if you prove that the asthma is/has never been an issue.
On a separate note, if the flying falls flat on its face I'll consider a career in the legal service... it seems to pay well enough!