Thanks again for the suggestions guys, its much appreciated
OK slight update - in addition to cleaning the earths under the bonnet (behind each headlight, battery tray, inlet, behind battery, bonnet) we've cleaned the ones behind the dash clocks that are mounted to the steering column support plus those in the the side of the footwells and the gearbox to chassis earth.
Looked for some in the book for the rear lights (as someone suggested this could be a fault) but they are earthed through the loom that heads towards the front of the car. Cleaned the terminals just in case though.
Suspect the stereo earths are those on the steering column support so thats them cleaned, will unplug the reverse light switch and try that. The headunit uses the original loom so will unplug that aswell and see if the fault disapears.
With the engine running, the moment you put indicators on the dash lights go dim and pulse in time with the indicators. When you touch the brake pedal they go dim as well and return to normal again almost straight away (even with foot still on brake). The rev counter also flickers either when brake pedal is initially applied or when indicators are turned on. The hazards however have no effect on either, nor do the wipers?
Any other ideas?