This is a general response to all the helpful people above.
Having spent the best part of yesterday trying out various combinations of devices and locations, I am going to admit partial defeat.
For the record, these components were 3 x BT Mini Whole Home Wifi disks, a TP-link WPA7617 Base unit and two TP-Link WPA7617 Wifi extenders.
With the BT disks, it proved impossible to get them to operate at anything above 40-45Mbps in any room in the house, except where they were in the same room as the Virgin Media Hub 4 router, where I could get 150Mbps+! After talking with BT, I have to admit there is probably an environmental problem in my house, the walls preventing better performance. With the previous BT Smart Hub, I was only contracted to get up to 60Mbps, so the actual Mini Disc rates were less significant.
The good news is that I have managed to get good results from the TP-Link remote devices. With one positioned in the corner of my dining-room, I can get a Wifi download speed of around 180Mbps. So I am going to use that device for Wifi downstairs. Bizarrely, locating it in in the far corner of the dining-room, about 12 feet away, I could not get more than 100Mbps, suggesting that my Ring Main (40yo) may not be as fit for purpose as I had expected!
Upstairs, I had previously located the other TP-Link remote device in a bedroom, which is roughly above the dining room, and used it as an Ethernet feed for a SKY Q Mini box, with the Wifi turned off. Turning on the Wifi (5Ghz only) on this device, I could get a download speed of c150Mbps.
I shall therefore retire the BT Mini disks and just use the TP-Link Wifi networks.
Once again, many thanks for your replies and helpful suggestions.
