Hi. I heen playing around again and have found the problem
On the set up I have always had the Ethernet cable running from and to both routers on LAN2 on both routers.
I tried connecteing the 'Flat' connection into the red WAN connection and bingo I now have maximum WiFi in the flat.
Many thanks for help with all the other settings.
I have PM'd you with the info on the setup. But glad you have it working now through a slightly alternate way