Firstly, many thanks to all who chipped in with suggestions and my apologies for not coming back sooner, but hopefully you can understand why
I spent a lot of time investigating all the suggestions but never did get round to implementing any of them (mainly because I know from bitter experience that I am more than capable of totally killing a network if I start messing about from a position of ignorance).
Finally, yesterday, I was just starting to pluck up the courage to start messing about with router settings etc when my teenage son came to me and said, "Dad, the wireless printer I bought for my bedroom isn't working properly"! Basically, he'd stuck a wireless printer in his room and tried to get it working by messing about with various network settings without my knowledge. A quick uninstall of that printer and a reset of settings and all is well again. I've even successfully returned that printer (because there seemed to be nothing I could do that would get it to work) and replaced it with another one that both works and doesn't impact the network.