1996 Rover 214Si (Bubble) Poor MPG

[Astra Dan]

Boss's daughter's car. Just on 40k. She's complaining of very bad MPG. Had a look over it and the plugs look a little dark, not nice and white (lean) like I'd expect a catted car to have. Discovered these things don't have ECU lights and I don't have gear to interrogate it to even see if there's any faults.
Measured the coolant sender and it was hundreds of ohms hot, when cooled a bit went up to just under 2k ohms, so that seems OK.
Measured the lambda signal, not really sure if it's ok. Generally a signal fluctuating from 0V - 1V is good, right? This is at warm idle:



Any suggestions welcomed. Thanks!