OK, just in case it helps anybody else:
Subsequent to that test, I was flying solo, and on the climb out there was a smell of burning. Solo. On my own. No examiner to hand control to.
But we all read the accident reports, don't we. So I remembered the one in which a pilot wrongly diagnosed a burning smell as an engine fire (it wasn't), turned the engine off, and killed herself in the resulting crash.
So I did nothing. At all. Whilst I thought about it. For seconds and seconds and seconds. (Well, it felt like a long time, anyway.)
And eventually worked out that it was the cabin heater, which hadn't been used for months.