Thanks for the tips Jeremy - it seems that basically the best way to get a crew place is to turn up at the marina and ask around! Looks like I'll just have to move to the south coast then...
Everone has their own point of view of course, but having spent a few years in the industry I would advise against spending the considerable amount of cash required on a UKSA YM course, which will probably only get you a job as a 50 quid a week sunsail skipper or a deckhand on a superyacht.
For that type of cash you could take a year long holiday crewing worldwide on deliveries, or simply as extra crew on cruising yachts. The experience / miles / sheer enjoyment you get doing this knocks the aforementioned course into a top hat pound for pound.
Afterwards you can take a week long YM course for 400 quid. You then have the qualifications AND experience to get a decent job
If you don't want to "work" (if you can call it that) for free, then the worldwide shortage in yacht crew will ensure you a job scrubbing decks on a superyacht (probably alongside recently qualified YM's) simply by contacting one of the many agencies advertising in Yachting World. Altenativly fly to Gib, Malta, Antigua, Greece or any of the other major yachting centres and ask around. You should have a job within a week.
Qualifications are always important, but what really counts in this industry is experience. This puts most fastrack grads and unqualified bums on the same footing for quite some time.
You could also buy a smallish boat and learn at your own pace whilst cruising around
But hey, everyones path is different and you may find a fastrack more suitible for the goal you have in mind.
Whatever you decide to do, there are a lot of options open to you in one of the most exciting, challenging industries in the world.
Thanks for the advice - I really had no idea I'd be able to do it the long way round i.e. get experience before the qualifications. It seems it's probably going to cost me just as much in the end whichever way I do it. I s'pose if I spend time getting experience I'll have a better idea of where I want to go in yachting before spending lots of cash - definitely a good thing!