In reply to Jeremy Foster:
Contacts are always useful - are there any friends, or friends of friends, on the team you're looking to join? An informal, quiet approach can work wonders (it's how I got into my local team).
But a more direct approach can be equally successful. If you can find out who organises the training schedule, a polite message stating an interest - with a one-line summary of your relevant experience - and asking if you could observe a suitable training session would be a good way to go. Volunteering to be a casualty demonstrates a genuine desire to be useful.
Is there also a social membership or fundraising volunteers section? Showing active support would get your name on any shortlist of candidates.
If you want to do it, it oughtn't to be much of a mission to get there.
Martin