Did you do anything in writing? And why are you letting them carrying on?
Step 1: Find cheaper insurance, set it up to change on X date.
Step 2: Cancel current insurance in writing, stating X date at which you want it to stop.
Errr, that's it.

If they don't act on it, then go after them as to why not when you have given them specific instructions, and demand a refund from when you cancelled.
If you knew your insurance premiums were too high, and yet let them carry on despite your protests, then there's nothing you can do, as you will be deemed to have accepted those terms. There's nothing unlawful about one insurance charging more than the other, they rely on people's inertia to do so.
Change as soon as possible, and learn from this that the only person you can rely on to look after your finances in your interest is yourself.
Insurance linked to mortgage is a really poor deal anyway, as you realised. Tip: If you have PPI linked to your mortgage too, you would do well to look into that too. I had my PPI, life insurance AND house insurance all bundled in with my mortgage up to last year. By switching to 3 different providers, I now save £80 a MONTH!

Yes, it took a bit of work to get it all changed to start with, but what a return!
Tip 2: In case of PPI/mortgage protection, always choose an insurance with a "back to day 1" policy, it is VERY worth it when it comes to big payments like a mortgage.
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