Slowly putting everything back together now. Had a good clean and refasten of the main earth points on the chassis and gearbox. Made no difference to the issue of all indicators going and dim dash light with left turn on stalk applied.
Did a good check of all the wires coming off the back of the binnacle and around the gauges etc - again couldn't find anything and no change in the error.
Have removed the dash light for the time being and am slowly putting everything back together, checking all the way. So far the harness loom to the front left has been rewrapped and everything connected and put back in place and everything still works.
Considering our Defender is relatively basic in the electrics department it's astonishing how many wires there are!