Many thanks for the additional posts

Had it back in this afternoon and the customer said the MIL came back on the next day

I checked the voltages sensor side similar to Daves waveform



Yellow = Front Sensor
Green - Rear Sensor

Engine Hot
Key Off
Key On....and battery voltage
Slight dip when starting
Full voltage present when running

I then checked with Amp Clamp (Pico)
0.5 amps at front sensor
0.0 amps at rear sensor

So check resistance again and this time
15 ohms at front
0 ohms at rear

So I think the sensor has now completely failed

New sensor on order from Toyota £158:00 plus vat

Thanks again for all the help, I certainly know more about 02 heater circuits now

Peter