Avoiding refund of deposit

I stayed at this self catering establishment for business for a month, they took a damage deposit which is supposed to be refunded within 10 weks after your stay, it has now been over 3 weeks, when I enquired, I was told there was an outstanding invoice which they have failed to produce in over a week since I asked them, which is no surprise since all invoices were paid a month in advance, they have since gone quiet, ignoring my messages.