After posting on a thread, I got a cheerful-sounding PM from someone commenting favourably on what I had said and asking some fairly reasonable questions. We had a few messages backwards and forwards, but it occurred to me that all the questions were based on bouncing back what I had said, and that the poster didn't come up with any original ideas. Also, there was no first name at the end of the messages, which I (perhaps wrongly) regarded as etiquette for PMs.
Anyway, I was pretty sure that it was a bot, and I know that I am not the only person to have had PMs from that "person". Apparently the name is now classed as a "former user", so perhaps someone has already reported it.
We all need to think about PMs from unfamiliar names - are they looking for information or am I paranoid?...