Keep on at them and write to your MP.
I had nothing but stress and grief, with letters and telephone calls being ignored while XP essentially did what he wanted, refusing to pay me anything.
So, I contacted my MP, I sent on copies of all letters and lo and behold, things started to move.
For the past two months I have been getting updates evey two weeks and now, finally...my XP has paid all the arrears he owes me.