Ah ha actualy did check this today, compared it to rear bank, sensor one, and spot on Mick!
Front sensor open circuit, are these the heater wires?
It also seems this is a zirconia sensor as when new one was fitted 0-800mv recorded switch.

One sensor going down seemed to really badly affect the other one, with a new one fitted the remaining ones activity was restored!

Thanks for all posts, phrom fill.