Update on this for anyone with similar problems. The problem was the 12v to 5v converter for my dashcam. I thought that I had ruled this out by switching the dashcam off to test it but forgot that the converter was still connected.
The only way to test it properly was to remove the (piggy back) fuse from the fusebox. When I did this, the "noise" disappeared when in the ignition on position as opposed to no interference in the accessories position. I changed the 12 - 5v converter to
this one and the interference has gone.
Apologies if anyone has already scanned my other post on this but it may help someone in a similar position when using the search facility.
The dashcam forums are awash with similar problems, so I was not alone!