EDIT:

Ok I just ran an earth wire direct from the battery to the tail light earth, had the same problem. Then ran it to rear indicators one at a time, same problem. The wire I used was not long enough to reach the front indicators, I will try that another time.

Another thing to add: if I leave for example one of the indicator wires on the rear left indicator unplugged, the front left indicator flashes at a faster rate as expected. The rear light also flashes at this same faster rate. This further points to a bad earth I think.

Any other ideas?

Many thanks

Harry