Unfortunately as Allianz have provided the Insurance cover in good faith, continuing in line with the original documents issued, you don't have any right to a backdated cancellation refund.
What you could do is write to the Allianz office that deals with the Insurance, with any proof of the phone being cancelled after 12 months. This is the Allianz phone number noted on Dialaphone site.
Allianz Insurance Schemes: 0844 391 4036.
Give them a phone call to sound them out, find the address to write to. My experience is that some Insurers did provide backdated refunds when evidence was received that cover was not required, but I know Insurers are no longer as agreeable.