@John ******
interesting John
When I left Canada my accountant told me I needed to sever all ties with Canada and my pensions are paid tax free in Canada but declared in the UK.
Perhaps you're getting 3 UK and Cdn tax allowance which I'm told you cannot do.
My accountant specialises in Canada / UK taxation, if he's got it wrong I'd live the evidence so I can reclaim £££
I pay over £5,500 in UK for my Cdn pensions plus my normal taxing the UK on all income above £12,000.