I agree with you in some respect Entwood, but there are some insurance companies who have a clause that if you make a claim you must settle the balance of your premium in full 
Admittedly, AFAIK, most of the big ones don't but some do
That's what I would expect, TBH. You have received the full benefit of the policy by making a claim. Why shouldn't you be liable to pay the full premium?
Another way to look at it... The premium for cover paid annually is calculated on the assumption that the money arrives in the insurance companies' coffers on day 1 of the policy. That money stays there for 365 days of cover, so that's 365 days the insurance company has to invest the
whole premium and make a profit out of it.
If the premium arrives in dribs and drabs over the year the insurance company has much less income from investing your premium.
The insurance company must make money out of investing your premium (in addition to the premium itself) to cover their administration costs, generate a profit for themselves and cover the possibility that you or other parties might make a claim.
If you paid your insurance by instalments, the cost wasn't greater, and you put 11/12ths of the premium in an investment on day 1, paid the rest of the instalments out of the investment when due, at the end of the year, you'd have made a profit. Wouldn't everybody do this if the insurance company didn't charge more?
So, it's hardly surprising that the insurance company calculate a higher premium on a monthly basis than an annual basis.
Whether the difference reflects the real cost to the insurance company or whether they are charging you a premium for the convenience of doing so is another matter, but it's not at all surprising nor, IMHO, a rip-off, that they do.
Kevin