I am trying to helps mate fix an issue on a 51 plate TD5 disco, the background is the car was driven through some deep water and cut out.

Upon restarting it will idle and drive fine as long as you stay below 2.5k rpm, if you go above that the engine cuts out and the MIL comes on. I have plugged my Faultmate and the only reoccurring fault is now Top Side Switch failure.

The question is what is the common cause of this fault coding being logged ?

Not good m8, that "top side switch" is an integrated part of the ECM(engine ECU)... when it happened to a friend of mine only a replacement ECU was the cure... usually these kind of internal failures are due to overvoltage/AC feed generated by a bad alternator diode pack/regulator or to a loosen feed pin/earth to the ECU, check the earths on that rail which is near the battery/jack compartment (especially the middle one).... if you are in UK is easyer for you to find an ECU repair shop and send the ECU to them cos it will keep all the injector and immobiliser settings then...or maybe you are lucky and the switch isnt fubar'd yet just the missing earth or feed ... also check the contacts of the ECU black plug

Thanks for that will take a look, guessing the big puddle hasn't done the ecu any favours.