Dear Tim, Yes - but takes some time.
Anyway, I tried this with most question types from the core that are worth embedding, i.e. computer marked, and they all worked fine! So then went back to the original question which had the odd behaviour and it was still odd. Finally, I wrote it again as a new question but with the same content and it now works fine! Bloody hell! There must be something in the old version of the question that causes a problem! So sorry to waste your time. But worth bearing in mind in case it happens again.
For information, I also tried a smaller range of WIRIS question types. They all render fine initially but then produce errors when trying to feed back an answer. I will talk more with WIRIS but thought it might be useful to know in case anyone else asks. Best wishes Dave