But, at this point, I do recommend engaging counsel. As I understand it, truth is not a defense against slander under UK law.
Edit: I meant "libel", not "slander". Also, I was wrong about truth not being a defense. It's just that you need to prove the claims.
The onus is on the person who said the thing to prove it is true ("guilty until proven innocent," since it's a civil rather than criminal offence), but as long as he has documentation/emails it's all good.
Indeed, I would be astonished if they sued him for this, it would not end well for them.
Disclaimer: not a lawyer, but an election agent.
(I just got a quote this morning for a what I thought would be simply "explain my property deeds", and it came out at £500...)
I really would not recommend getting involved with libel law on a pro-se basis.
It most certainly is. Defence #1, in fact.
Under US law, the burden of proof rests on the plaintiff.