Thanks for the responses guys... just to recap the trail of events the symptoms are identical to what happened before the Forge DV was installed. The very first set of occurrences turned out to be the stock DV which had ruptured its rubber seal. Apparently a very common fault.
At this time I had a couple of faults logged - something around low or under boost.
More recently, it's happened with the Forge DV. This is despite being led to believe that the Forge DV is pretty much immune to failure due to its mechanism and construction.
As my recent power loss happened at a leisurely 50m/ph on the M1 through road works I can't see it being an over boost issue. Also, this time round no faults have been registered.
Recently I have had a HPFP and intercooler upgrade, and also the cam follower changed as a precautionary measure whilst everything was open. If it's not the Forge DV I suspect its a leaking boost pipe - as the vacuum pressure on the DEFI gauge has dropped below 20. Being able to see this information alone in my opinion has paid for the gauge...it' a great diagnostic tool.
Current plan is to visit TTS this Thursday so they can have a look as they originally carried out all the works. I'm also considering the latest rev. of the VW diverter valve. Some people reckon it's actually better than the Forge one. If I can get this to work with my DEFI gauge then I may go down that route.
In any case I will keep you all posted on progress...and if you guys could do the same that would be great.