Quick update on this - just fitted the Bosch sensor from the link above which was £47 inc postage... and the issue is solved. MPG shot up from 28 before to 39 (?!) on the motorway, a pretty massive improvement...the non-genuine ones are complete rubbish!
At the same time I fitted a drivers side driveshaft from J & R which was a massive bargain at £50! It even came with both cup and non cup ABS rings, and proper plastic boots.
A good result!