I realise this is very

but someone here may just know the answer...?
I have tried searching the relevant forums but have drawn a blank.
I've been getting my car ready for an MOT over the last few weeks but have one remaining problem.
The speedo doesn't work, has not done so for four years but I've got away with it. But even so it would be great if it could be fixed. So far three garages have failed to sort it out over as many years.
I have been using a nifty little gadget that picks up Satellite signals and displays my true speed, unlike the guesstimate of the original speedo.
So i do know how fast i'm going...
But is it legal, and should it comply with the MOT requirements. Common sense would say yes....but...we don't seem to have much of that these days....