Embarrassingly, immediately after posting the original message I got called abroad (sounds exciting - it wasn't!) to a place where I had no internet access so I've been unable to read or contribute to the posts.
Thanks for everyone who has commented. My problem is unresolved. Interestingly I took a Windows 10 laptop with me and put BIAB and Realband on that to have something to amuse myself with when working. Wouldn't you know it: exactly the same problem.
Time to raise the issue with support, I guess.