Thanks to all who replied with different solutions.
Joel Walker had the answer ----

  ( sounds to me like the plugin isn't perfectly fitted to the lighter
socket,and you've got some power going back into the ground circuit
(which includes the relay and the oil pressure light).

you might have to buy a standard (non-vw) lighter socket and wire it
into the dash power/ground to get it to work properly.

good luck!
joel )

On inspecting the lighter socket, I found VW had the +ve contact inside
the socket extending via 2 blades further down the side of the socket
than a standard socket, and these were making contact with the -ve
contact of the handsfree plug.
So I can avoid the power feeding back, by orienting the plug to avoid
the blades, but as Joel suggests, I have installed a standard socket to
prevent further problems.

Thanks again!

Bob Turnbull

91 Syncro