So I've got a knocking coming from passenger rear, assumed it was a wheel bearing but mechanic wreckons it's the spring knocking on the shell because the previous owner put lowering tein springs on the standard shocks but my I'm not sure because it only does it on that one corner and I've had the car 3 years and it's only started the last couple of weeks, surely if it was because of the springs it would have done it since I owned the car? Any thoughts 

Well if anything hitting against shell you likely have witness mark from it .
If it is coming from the strut it could be strut spring or top mount .
don't overlook other suspension areas such as drop link or bushes.
wheel bearings don't make knocking noises as a rule ...
Best way forward is good visual inspection and if nothing obvious pull the strut for examination .
What could be happening is the strut is bottoming out with the lowered spring now it few years older and settled lower .