and... this happened:
Moral of the story:
Do *not* jack up where Mr Datsun tells you to, the thing had just rusted and my car has only done 79k!
Jack up somewhere else such as the diff and don't forget to use axle stands!
Namelezz on here was kind enough to pop over on the weekend and help me replace my left rear break pads, he had a look and said its definitely an mot failure (any sort of hole like that within 30cm of suspension components is apparently). His mate reckons its weldable so will have to go and make a trip next weekend and let him have a proper look.