Joint mortgage after separation

[Bizeli]

I had a joint mortage with my former partner whom I split from several years ago. I left the home, and he stayed in residence. My partner has not been paying his share of the mortgage since I left, which means the mortgage is now around £18K arrears. I have continued to pay my half, as I realise I am jointly liable for it. He refuses to sell the house, or engage in conversation about sorting this out. The mortgage company don't seem to want to foreclose, possibly because I believe we may be in negative equity, and so a sale wouldn't clear all of the debt. This is affecting my credit rating, meaning that I'm unable to get a mortgage for myself. I've been quoted from a solicitor that it will likely cost around £5K to sort this out, which I can't afford. Can anyone please suggest the best way to extricate myself from this situation.