Russell Hobbs- no after sales service

I I bough a Russell Hobbs microwave in August last year, It now does not heat up the food.

I have emailed Russell Hobbs to inform them of this but they are not very helpful. I bought this from a 3rd party. He was selling Russell Hobbs products and I supported him. It was brand new and boxed. They now want a proof of purchase which I told them that i do not have. I have furnished them with the invoice, but they demand the proof of purchase.

I want them to fix it, I will pay for the repairs.

This is shocking Russell Hobbs is expensive.