Thanks Moravcik, though its not the medical exam itself that is the problem at the minute, I passed the examination at Swanwick no problem... but I've got a history of conditions involving chronic headache (possibly migrainous, but can't say for sure, almost every doctor I've seen has told me it is something different).
The guys at NATS said that they themselves aren't bothered by it - the symptoms are very stable and consistent and as far as they are concerned I passed selection despite the condition so it obviously doesn't hinder me in a way that should present any significant issues - but they don't think that the CAA are going to be quite so accommodating.
At the moment I'm waiting for the CAA to revue my notes and make a decision on it, but I'm not feeling very confident it's going to be good news
Bit gutting really as I'd looked at the medical requirements that are on the NATS website quite thoroughly before I applied and really didn't think I was going to have a problem.