I was talking about this type of thing yesterday
I'm with Very, and so far, touch wood, they have been perfect with any returns etc I have had
After giving my details recently for an order I needed to return because it was damaged, the first thing out of the customer service operator's mouth was 'I'll just refund your delivery charge' then he arranged for collection and reorder the shoes I wanted. Shoes arrived with the collection courier
A bookcase I ordered that arrived damaged was also immediately reimbursed to my a/c, collected when they said it would be and a new one delivered on the same day
If I were you, I would be thinking a recorded letter sent with instructions on what to do within a set time scale 'or else' type.
They really are dragging their heels on this one and have most likely lost a customer to boot and are not complying with DSR rules either