After much searching on various forums I found a thread on an Ozzie site with all the same symptoms and the above solution. Apparently it's not uncommon for the sensor to start malfunctioning when hot on vehicles with around 75k on the clock. After spending just £38 on Ebay for a Blue Print replacement and a five minute swap, hey presto! Running sweet and much improved performance.
My local garage wanted to change the ignition coil packs at great expense as it was the only possible cause as far as they were concerned.
Just wanted to share the knowledge.
Happy days!
