Hi Ian,
The problem is more than likely with the PCB, I have just changed mine (after the same problem as you) and changed two sensors , parts were £50 PCB and £20 for 2 sensors and everything is fine now.
I bought a repaired pcb which is a fraction of the cost of new from Adlink in Southall.
If you need any help or are local then give me a shout.
shahid.uk@gmail.com
Regards
Shahid Aziz
February 2009