Further to my post above, the problem was getting worse, especially with the offside indicators. The hazard lights, however, were never a problem. I found some posts on various fora which all pointed to bad connections on the rear lamp clusters, especially the earth. I removed the lampholder assemblies and there were signs of overheating at the earth contact. So I drilled a small hole in the earth bars on the cluster and attached a wire which went to earth on one of the cluster mounting screws. Result? Everything back to normal.
Thus, the fault had to be a high resistance on the (standard) earth connection which was sufficient to confuse the lighting control module (which controls the rate of flash (if my info is correct)), but not enough to trigger a fault.
