I have two Mk1 Octys with the 110BHP ASV engine. One of them I've only just bought (has supposedly just had cambelt and water pump replaced) and it has a rattle coming from the belts area. I've noticed what I think is the auxilary belt tensioner jumping around a bit more than in my other car (I've circled it in the attached picture). Could this be the source of the noise? Should I be worried about it?

If you put your finger on it and the rattling stops you have found your source. The damper on them can become tired. I changed mine about this time last year, as I was changing the pulley on the alternator due to it breaking off. I concluded that mine was pasts its prime. (160k-ish?)