Yes , all the pension that has been paid since your Mum's death has to be repaid to the insurance company. Failure to inform them of the death could be regarded as fraud. The bank must also be informed, a dead person can not have a bank account. If you had informed them of the death the bank would have done most of the work informing people like the pension company.
What about your Mum's state pension is that still being paid into the bank?
If so you must inform the DWP as well , unless you start doing something you will end up in a lot of trouble, as I said you could be charged with fraud! They are not going to be happy that they have been paying a pension for over a year after the death.