poor old Rover.... (Diesel non-starter)

[T-Cut]

Have you tried simply allowing the ECU to dry out thoroughly before reinstalling it?
If everything was OK until you splashed water onto it (cornering) and until it got flooded completely with rain water, maybe it's intrinsically OK. Mechanically they are pretty robust and will withstand submersion provided they don't short circuit too dramatically. Try putting it somewhere warm for a day and see if it comes back to life. Worth a shot, no matter how long at this stage. Flushing them through with isopropanol is also a favourite way of removing debris left by rainwater on the electronics.

TC