Just posting an update. After getting a mechanic friend to dismantle everything to get at the sensors, it was apparent that the wiring harness to both sensors was badly corroded, as well as the tag in the sensors. While I may have got away with just a harness, the amount of work to dismantle again would not have been worth the risk. Bit the bullet and bought 2 genuine knock sensors from Lexus as well as wiring harness, gaskets and a bypass hose at £560 ( plus £275 labour and new coolant at a mate rate ). Car is running really well again with no errors even after a few weeks.
Thanks to everyone for your comments.