water .. yep that makes sense ..
I've formed a bit of a theory as to what might be happening .. yesterday I removed the pre-cat from my exhaust system. I was ultra careful not to get any debris down into the main cat, and vacuumed out the pipe before re-fitting. I think maybe since the exhaust gas hitting the O2 sensors is different, the ECU is re-learning and may be getting confused a bit.
Now today I drove about for 3 hours, giving it a mix of high speed freeway driving and low speed suburban driving - and I could not re-produce the fault. So I'd interested to hear about other's experiences with removing the pre-cat and if it produced a similar fault to mine.