Odd Lambda Sensor Behaviour

Took her for a test run this morning.   Drove to the surgery (5 minutes), turned off engine but left the data logger running while I was inside (5 minutes), then drove back home (5 minutes).

This was the lambda trace:

You can see it behaved itself on the way there, but when I restarted the engine to come home the sensor was so slow in reacting that the eCU gave up and went open loop for the whole trip back.  I have seen this before but never reproduced it this cleanly.

I now need to figure out why it is doing that.