I've booked my car in for a service tomorrow and I was offered what I think I heard as 'fuel tank cleaning'. It costs £12.95 and it's designed to improve fuel efficiency/economy.

Anybody know what this is and is it worth having it done?

They have most probably offered you a fuel treatment like a additive they put in the tank. It cleanes your fuel lines/injectors.

I used it every 3mths on a full tank - beware of cheap brands go for a well known brand like redex.