O2 sensors

Hi there.
Bit of a strange one here, guys.
2007 Vauxhall Corsa 3cyl. Petrol.
Came in for an EML: rich code. O2 Sens 2.
New exhaust. New o2 sens 2 already fitted.
Checked scanner- sens 1 good after a few secs, sens 2 stuck at 460mv. If I hold the revs at 3k or so for a few minutes the sens 2 slowly comes to life. Sweet, it's a defective heater in the sens 2. But sens 2 is already new.
I checked 4 wires a sens 2. Ohmed the heater with sens 2 disconnected and it has continuity. Checked wiring with it connected: earth good, 14v on one heater wire and pwm 14 - 0 on the other. Signal wire good. 450mv (on scanner) disconnected. Jumps to 1.3v if jumped to power and goes to 0v if jumped to ground.
Sod it I'll try a new sensor anyway - nope. New one is the same.
On road test, because the revs are high the sens 2 works but if I hold the throttle to the floor for 5 to 10 seconds (high gear so i don't blow the engine) the sens 1 doesn't go rich, it just keeps going above and below 450mv at the normal rate of steady throttle or idle.
Long and short term are around 0.
If I put acetylene in the intake the sens 1 goes rich and with oxygen it goes lean. So the sens 1 is fine that way but not when I hold the throttle down hard on a road test. It just jumps above and below 450mv.
Any ideas??
Also, when I come off the throttle if I rev it (in the workshop) for a few seconds, when i come off the throttle the two sensors read absolute 0v for a second or so. I thought they should never read totally 0v unless they are being grounded, which shouldn't happen. (See pic). Shaking wires did no difference to the signals.