Mediation broken down- what next?

[AllInTentsWithPorpoises]

So long story short. I'm divorcing a narcissistic arsehole. We agreed a mediation settlement and mediator drew up the paperwork. Ex then changed his mind 6 weeks later after going awol and not giving his comments on it. We are currently at a stand off. It looks like mediation is over. What options do I have now apart from court? Am I right in thinking mediation via solicitor/ solicitor to solicitor/ arbitration? What are the pros and cons if these? Basically ex decided he wanted to give me around £20k less than we'd agreed and now he won't budge.

File @court but send at least 4 letters 2 weeks apart stating what was agreed with dates and time frames and asking for a response

Wouldn't arbitration be cheaper/ quicker than court? It's only for the financial settlement, nothing else.
He didn't sign the memorandum of understanding, he went silent for just over a month after the mediator sending it, despite us both chasing him. Then emailed to say he was no longer happy with it. He made a revised offer with split more in his favour and is now refusing to budge. I have emailed twice, including the mediator, but still won't budge. I thought arbitration might be faster than court and still comes out with a decision he can't appeal/ go back on.

@AllInTentsWithPorpoises How are you doing? Any news?