Held boost figures in 4th and 5th gear throughout:

Cheap gauge 16 PSI
Falkland reference gauge 17 PSI
MAP sensor ouput 4.0-4.1V == 17.7 - 18.8 PSI (quality analogue multimeter, but tricky to read well on the move!)

So the cheap gauge is under-reading by around (say) 2PSI compared with MAP sensor. Not surprising then that I was hitting (soft) fuel cut out at just over indicated 17 PSI as the MAP sensor was probably seeing 19.3PSI (PPP's cutoff).

It makes no difference whether the gauge is off the manifold's spare port or off a T piece in the MAP sensor line.

Whatever the boost **really** is, now I know I can aim for peak boost staying under 17PSI and not wonder where the rest is and why I can't get it smoothly. Roll on the Dawes device and some more fiddling.

Basically, after a lot of fiddling with a boost gauge I think I was almost as well doing it blind before where I just went for small increases in wastegate restrictor size until it held boost smoothly and felt fast! A reading of 18PSI peak boost may make you believe you are OK but you are actually peaking at over 20PSI.

If I am really now holding 17-18PSI with PPP+DP that does not leave much room for a peak in boost without occasionally finding the cutout.