I've just got the car back from V12 once again and I'm happy to say the boosting issue has been resolved! It was definitely a software issue, though I did change a good few pipes that had been split.
It could potentially be worth taking it back one last time for a final tweak to the map, but overall I'm happy with the service V12 provided. I'm also still not ruling out a faulty n75 as the boost delivery is on and off at times.
Thanks for all the help