Replying to myself!
For anyone interested, I had read on other sites that there may be a reed switch within the speedo which would effectively cut power at a certain speed. Well I've been and looked, and on my model all the wires into the clocks connect to Bulbs for either warning lights or backlighting the dials.
I'm left asking the question if the only further route forward other than the DEP pipes is to source a CDI unit from outside the UK for a non restricted machine.
Any help or advice (No, really ANY!) would be appreciated.