Have you checked the tps voltage? I had a lotus k series with exactly the same problem. Found it by checking tps voltage at idle. Should be a steady voltage (about 0.8 volt if i remember correctly).
Mine was up and down on tickover without throttle moving. There is a spring inside the sensor and that had failed allowing the sensor to shake. Changed tps got it off an old metro in the scrapyard and fault was cured.
Also worth doing a compression test as have had another similar problem where valve seats have warped when engine has been overheated very common and will cause a similar fault and flag up o2 sensor faults due to raw fuel etc going down the pipe. head back off recut valves and seats and back together usually sorts it.