[CMOTD]

I don't think the interference was with the sensors but with the I/R overhead lights. The sensors are wired with screened cable direct to the parallel port on the laptop and the wiring is nowhere near the 12v DC supply for the LEDs and light bridge. Presumably there was some sort of overload or voltage drop which caused the the overheads to flicker and register false laps but why it only affected one lane remains a mystery. In fact the whole thing is beyond anything I have experienced before, the system has worked faultlessly for several years and LEDs draw a miniscule amount of power so the wiring was perfectly adequate for the current drawn. Anyway, lesson learnt for the future - don't put different functions on the same power supply.