It would seem that my GPS unit was fitted before the authouritys had given any thought to the matter what so ever.
So the correct answer to the question is that you do need a flight manual suppliment approved for your instalation.
Bendix/Knig produce a genaric FMS for all there units and if you can find an EASA FMS for your unit then that will do and all you will need to do is have the instalation checked to be IAW the TSO and submit the FMS for approval for your aircraft.
I cant see it all going the wrong side of £150 as long as the unit has been properly installed.