Quick update,
Popped down to Statller and decided to give the Turbosmart diverter valve/blow off valve a go on a more permanent basis. This DV/BOV is designed to run as a normal DV at part throttle when lifting off so that the car runs smoothly in traffic etc and at high boost/WOT when lifting off acts as an atmospheric BOV. This is a great piece of kit and a proper "boys toy!"
The sound when it is letting off from high boost is great.....loud without being deafening. Not something I am used to, but it is needed to help expel all that excess boost without killing another throttle valve.
One of the good features is that you can decide if you want it to be entirely a DV of part DV and part blow off. It holds boost very well too. Photo below is the best I could get off the phone camera......31.8psi