Just to clarify matters chaps..
1, The Sony cd/radio was fitted in the car when i bought it a few years ago.
2, I had the seconhand Vauxhall VDO headunit fitted by an auto audio technician a few months after i had bought the car.
3, Both worked ok with no probs with volume control.
I think for my own peace of mind i will visit a bona fida installer and get a quote for a proper new installation of a new, either the JVC or Kenwood, unit and all the sundries.
Thanks for the comments.
Cheers, Rob.