I'm an insurance salesman (booo,hissss). Each insurer will have a different procedure and you need to call your provider to find out, but like others have said, expect some hefty charges.
But try calling as if you are a new customer, using a fake name, address and vehicle reg. Get them to give you a quote. Once you have your price make sure you asks them loads of questions about cancelations. Try and get them to state a standard fee they charge each customer and, if possible, try and get them to state how long this has been their standard charge.
Here the REALLY IMPORTANT part. Make sure you record this whole call and take the operators name.
Then, if they apply ridiculous charges, present them with this phone call. If they deny the facts stated in the call, then follow the complaints procedure.
It is very expensive for them if it reaches the stage of the financial ombudsmen`s involvement, so you will stand a good chance of getting these cancellation costs reduced to the bare minimum.
TIP FOR THE FUTURE - If your insurer/broker is willing to tell you their standard cancellation fee before you buy the policy, then go elsewhere.
For more go to
http://www.squidoo.com/tips-4-car-insurance