Update - whether you want it or not........
Since I started this thread, internet speed at laptop, tablets and phones been fluctuating this past few days - didn't seem quite right. I'd been getting over 500 at my laptop (very good) but something had gone seriously wrong in the chain - still working but not right.
Made a call to BT to find out what was going on. Decent Geordie (or could be a Mackem


) got involved and sent me a link so we could both check the incoming service speed from the cabinet and actual speeds at the devices. Incoming speed at the wall modem was good at 949 meg, but the devices were all over the place from as low as 18 up about 55.
Now when I upgraded to FFTP I already had a Smarthub 2 and the guy at BT said no need for a new one. The one I had was for FFTC but can be converted to FFTP by configuring port 4 to a WAN input. When the Openreach guy arrived to install the fibre service, we made the switch.
However, and this is the big however, the router conversion is a compromise - they look the same but they're not. The guy I spoke to today said when I upgraded, I should have been sent a brand new Smarthub 2 specifically configured for FFTP as this issue is not unknown.
He's arranged that and should get the new hub Monday/Tuesday. There will be a further update to follow..........