Hi y'all,
Sorry for not replying earlier and I'm grateful for your tests. Having read all you've done I cleared the cache on my iphone's Safari browser and tried again and the You Tube search now worked as expected on the phone app for the B2 in the kitchen. I then tried again with the phone app for the B2 in the lounge (which has a different address of course) and no joy. Using the pc I could get the search facility to work on this B2 using Edge, Firefox and Chrome but I could not get it to work from the phone.
It took a day for the penny to drop - the pc is adjacent to the router where the wi-fi signal is very good. The wi-fi signal is also good in the kitchen. It is, however, less good but adequate, in the lounge. So I moved the lounge B2 to the office, adjacent to the router, tested again and suddenly everything worked fine. So a weak(ish) wi-fi signal would appear, on teh face of it, to have been the root cause of my problem - except that, when I now took the B2 back to the lounge the Search facility works just fine from the phone. It's almost as though it needed to be kick started with a good wi-fi signal to work with a less good one. But that can't be right can it :-) Oh the joy of messing around with electronic stuff (not!).
Paul