Thanks for all your help guys, I tried an N75F from my friends AUM last night and I can make over 20psi boost again Would seem that my valve is just slowly failing.Garrett did a nice £30 mechanical boost gauge that I had fitted in my Fabia once upon a time.
Eventually it will tail off as the turbo runs out of puff, I just need a new cheap boost gauge that works with no fancy sensor crack, even if its a psi or two inaccurate as 10psi with and engine off is not a good boost gauge