.....breaking news....
We have sound - yay!!!
With all thanks going to member Neil_f who met up with me today and weaved some VCDS magic.
The problem and the solution was as summarised in various posts above. The RNS510 had been coded for a car fitted with a Skoda sound system with DSP, and also with active speakers. Changed these settings to no sound system, and front and rear speaker channels. Save new code and reboot the RNS510 using the 3 front buttons. Switch on and glorious sound ....well Radio 2 came out the speakers.
Very happy! Now just to get a dog guard fitted.