Thanks to everyone who helped me out, still not fixed but looks like the problem has been identified.
I have to say a massive thanks to N13KKF from the r32.oc. He supplied the RNS510, great price, excellent packaging and all round top man, going out of his way to make sure I had it delivered on time.
It may seem a bit daunting to send him money without knowing the guy but he has been nothing but superb.
Check him out if you're looking for an RNS510.
Jim