From the BBC:
"..Richard Coyle plays Jeff, who has heaps of advice about dealing with the opposite sex, but very little practical experience - until now. In fact Jeff is in a serious relationship with his boss Julia, who he first got together with in the stationery cupboard at work."
To me, Jeff Murdock was the best character that Coupling ever had. The completely random and insane comments, lines & gags this guy would come up with..
From the beginning of the show, Jeff was never comfortable around women. Well, he wasn't comfortable around anyone, really (except maybe Steve and Patrick), but his exploits with the opposite sex have led to some hilarious moments throughout the series.