Daft question, but when they last worked, it was after the engine was at running temp.
could it be anything to do with the dim dip resister which i think is the off side wing ??

With the help of an RAC man and his superior electrical test equipment we isolated the fault to the supply from the main fuse under the seat to the switch. So a temporary but totally safe fix has been to remove the fused link (no 6) and run a new 30 amp wire from the live rail in the fuse box, through a 30amp spade fuse instead straight to the switch. (ive cut the brown wire and made the new connection on the switch side of the connector leaving the loom intact for when I can trace the actual break or short). and then there was light !
Thanks to all who offered their help and suggestions.