They tried the same with me on my 4be. Had a policy with them, had no problems, then they suddenly decided that I had taken the policy out with them as a female and had to pay an extra £1,300.

Unfortunately for them, I had a printout of my online form (as I always do as a backup) which proved I had it as "male". Also, all my communications were addressed to "Mr I. Southam", not "Mrs" or "Miss". I never had to pay the amount, and cancelled the policy out of point of principle as they would probably found a reason not to pay out should I have an accident anyway. Mine was paid annually though, not monthly.

I would take legal advice, even if it is CAB, and do it quickly, like tomorrow.

If not, speak to Tony @ Greenlight as he may be able to unofficially advise you.