Thanks UTY.
I have MP60s with S300 fitted at the Leatherhead fitting centre, but since then I have changed my swing to strike down a bit more. My local pro measured the 6 iron spin at 6600 rpm, which he said was about 1000 rpm too high. I have also hit MP52 and 62 in a PX 5.5 and although they seem fine at that flex, I would be a bit more anxious about whether I could flight a 4 iron off a downhill lie.
I had some work done on woods at a specialist clubfitter who suiggested the KBS might work, which they said was in between S300 and PX in spin. This place can set me up with a set of KBS, fully fitted to a high standard, but it would cost about 50% more than the MP62 black nickels with PX.