I redone the test after posting the question and made sure i had clamp right way round and it was ok and is now reading ok. It must of been something i did? i removed negative lead and put a jumper cable between post and terminal and took reading of the cable. Does the length of jumper cable affect the result or not?
cable length wont effect reading. if you have to use a smaller thickness wire inbetween battery terminal and clamp make sure you dont interrupt the vehicles power supply as this may hide the fault your testing for
The switch on the amp probe is rather touchy. It's easy to slide it and have it between the range detents and not realize it. This makes it behave very strangely. Make sure you have the slider switch firmly located at the selected range. Also, the battery can be low and not illuminate the low battery indicator but still cause bogus readings.