He denies this but hasn't sued the person making the allegation'
He can't sue the person who made the allegation. It was an MP who made the allegation in the House of Commons. This means that the MP is protected by Parliamentary Privilege which allows an MP to say whatever they want while in parliament. Very important for allowing MPs to have proper debate without interference.
However, having said that. I definitely think he did accept money from Russia (the truth is also a defence against being sued!). He appeared on Russia Today (state propaganda) a lot.
sab