In reply to Adam Perrett:
You do have some cover through the BMC, but it is limited to when you are purely consolidating your log book by taking groups out. If you are acting as a casual, freelance or volunteer for someone else then it doesn't cover you. I checked this out with the BMC a couple of years ago, for a similar reason, but please do double check with them yourself to confirm.
There are a fair few companies out there that offer Freelance Instructor insurance. Perkins-Slade have a deal with the MTA so you can get preferential rates. Not sure if you can you join the MTA though until you qualify, but I might be wrong. Alternatively, you'll need the company that you are working for to include you temporarily under their own Public Liability policy.
Do a bit of research and confirm all this though - I'm pretty sure that this is the way still still works with volunteers, but I could be wrong.
Good luck with the award,
Si