There may be better solutions,but this is what I have done.
Firstly,carefully remove plastic bung from centre of headstock :
to leave a big hole !! :
To mount a RAM ball,I used a threaded rod and ball I had from a previous bike and added some larger stainless washers and a couple of rubber washers to protect the bike :
So the bits will be fitted like this once on the bike :
So that's it-just fit to bike,taking care to centre it all up before tightening :
You can then just fit your GPS cradle and arm :
(I'm still waiting for new Garmin Nuvi 550 to arrive in post).
All very straightforward.
The RAM stuff can be bought here
and the threaded rod and washers I got from my local hardware store