Something does not seem right here
Firstly if you had an accident and did not report it I thought that the insurance company could not do anything until you had reported it, as it was their word against yours, (I would remind them that you are the customer and that they should be working with you on this and not against you, I bet you have just renewed your policy with them)
Does anyone else have access to keys for your car? maybe they borrowed it without asking!!
Also what really concerns me is that how has someone got all your details and gone to your insurance company and said you had an accident, I thought that there was such a thing as data protection.
You said that you called your insurance company, did you use the number on the top of the letter that you received today? (just thinking it maybe another scam). If so try using another number that is on your policy.
Sorry if I am stating the obvious here.
Anyhow good luck and let us know how you get on