Going through a similar(ish) thing where 2 years ago a massive lotry drive into my Zed, pushed me into a curb, wouldn't settle 50/50 said take me to court if you like, got paid out completely but now the lorry driver is claiming for injuries because the claim was actually settled 50/50 just my insurance decided they coukdn't be arsed to chase it and just paid me out, leaving the door open for the scumbag to then claim.... funny because he couldn't wait to get back to work when we were exchanging details.
Just another reason why insurance is going up because everybody thinks they HAVE to claim for injuries because everone else does...
Hope you get it sorted, very difficult without and witnesses, I would say dig your heels in and if you have legal cover say you will go to court, you may eventually keep your no claims and be blame free like me but beware you may still get dragged into a legal battle over injuries