Hi Kevin, thank you for your thoughts. I did check pins 4 & 5 at ECU connector, as advised, and these indeed did show 700 ohms, and when I unplugged front right sensor to loom connector they went to open circuit. The Tech2 had said it could not see front right sensor. I saw no reason to doubt the other sensors were connected OK.
I cannot imagine why the wheel sensors should record different speeds if they are working at all. The tyres are an assortment, all nominally 205 65R15. I have run on assorted tyres for years in all my cars, including mixing sizes on occasions, without ever upsetting the ABS systems.
As suggested before, I wonder if it is worth while swopping the ECU again (the one now in the car is the original one) and asking the Tech2 what it sees then, with a different front right sensor and a different ECU, and having established connection between front right sensor and the connector socket, and having different aluminium wires from ECU connector plug to PCB.