hi,i am changing my belt tensioner ,is it just a case of bolt off bolt on ?.

Unlikely, I'd set aside a good few hours for it as you never know what bolt is going to be an arse to get off. If you're changing the belts too then the air compressor belt can be a pain to get on and off.

If you have the time dismantle as much of the front end as you can to make access easier.