Canceling life insurance set up through mortgage advice bureau?

Last year in September me and my ex partner set up life insurance through mortgage advice bureau (third party seller )when choosing a mortgage with them. This meant they would waiver a £200 fee for setting up the mortgage and all was good.
Last month me and my partner split and I could not afford the monthly payments on the life insurance anymore as I was now having to run two homes until I was bought out of my mortgage. After cancelling my direct debit for the life insurance I was contacted by mortgage advice bureau who said pru protect (the insurance company) had changed them £900 as I had cancelled my insurance and the mortgage advice burea want me to pay them £900 which was taken from them. How is this possible? After looking through the underwriting on pru protects website it states I can cancel anytime and my cover will stop. It says nothing about a cancelation charge. Why does mortgage advice bureau feel they can charge me £900 when I'm in my right to cancel? Any ideas or am I missing something? As you can understand this is the last thing I need when sorting out a break up/selling a house and going to work.

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