RESOLVED!!
I replaced the PCV valve with one from the dealer and the vacuum has dropped to from 13inHg to 2inHg, just where it should be. Interestingly the 2 aftermarket PCV valves which I had tried both gave different vacuum readings but both were far too high. So do not buy a D2P autoparts or a Febi aftermarket PCV as despite appearing identical to the original and suck/blow testing in an identical way to the original they result in far too high a vacuum!
Thanks to the help of all the folks who contributed.