I don't think there is a limit, they do need to be on the same electrical ring though, so you can have one downstairs and the other upstairs, there's more info on the BT Shop page which might help you, see BT Broadband Extender Flex 600 Kit
If ordered via BT you will also have a 14 day cooling off period, so you won't be stuck with them if they don't work as expected.
Mine is about 100 yards away. Just trying to work out whether this is outside spec or faulty.
The limit for pure Ethernet is about 100metres, so I guess you are probably pushing the limit for Ethernet via mains cable.