Thanks @kongondo
As you say, a similar issue, although subtly different. The thread leads off into a lot of technical stuff that's beyond me, but also raises the possibility that the problem is the version of PHP, 7.2, and that reversion to 7.1 deals with it.
My host charges extra for supporting old versions (ie before 7.2), so I don't want to even try that. Instead, I have tried moving up to 7.4 and carrying out a fresh install. I get exactly the same result.
So right now I'm a bit stuck. I'll do some more searching - my initial search failed to throw up the thread that you kindly pointed me at, so I clearly wasn't doing it thoroughly.
cheers