Can connect to internet with phone but not with laptop

You did not respond to my #29. We do not know what cable lengths are involved but cable performance does diminish with cable length.

Sorry Fixer - see below. I got distracted by another suggestion. Anyway - SUCCESS! After checking cables (100 M and 25 M) for continuity and I tried again, first resetting extenders to factor mode, but found that mywifiext.net didn't log in to the Netgear. Neither did mywifiext.com, but 192.168.1.150 did. This time I was able go through the setup process on both extenders (naming networks NCH_EXT2.4 and NCH_EXT5, and OCH_EXT2.4 and OCH_EXT5 so that I could see what signals I was looking at) and was then able to log in to either Netgear AP with laptop, tablet and phone at either speed. I also was able to login to the modem (NETGEAR_EXT) and found it switched tot he AP ones as I moved to the halls. So all is now well. Except for Hive (which may or may not mean anything to you - it is a UK means of controlling central heating remotely). Tha will not login and after various attempts and tests I suspect the hub has a fault. One for Hive forum!!
Thank you all for your patience and help. I've learnt a lot about networking, but still think it is a black art taught at Hogwarts.

Well done!! You've left me slightly puzzled as each extender has to have its own IP, i.e. they can't both be 192.168.1.150 but looking at the connected devices in the BT router (Smart Hub 2?) should clear that up. I always use fixed IPs and address devices by IP rather than by name so do ensure the extender IP addresses are fixed so they never become inaccessible again.

I think that over the course of this marathon a lot of the time I was doing things by numbers without really understanding what I was doing. I know that at one stage I did get mywifiext.com to work, at other times I didn't. The .net version I never got to work. The IP version worked every time. (Don't ask me to explain why Fixer). Maybe I used IP with one extender and .com with the other! As I said, I was frequently confused. Anyway, I'll have a look at Smart Hub 2 tomorrow to see what connected devices I see.
Then I'll go and pester the people on the Hive forum!