Many thanks for producing the video. It helpfully showed I was following procedure correctly.

Couldn't find a 'temp' or 'tmp' file in BB root directory, or anything similar apart from a midi one that had a name with lots of exclamation marks, which I deleted. Not surprisingly it made no difference.

What I couldn't see in the video was any information messages, with either yellow or green backgrounds and wonder if they were turned off? Now reckon that Wavs are being produced correctly and error message is a spurious one. Think I'll just ignore it in future!