I changed two settings - it could be either or both.
In "Advanced - Configuration - LAN - Wireless"
I changed the mode to "b+g+n" rather than "g+n" - it's a "N" extender so I doubt this was what fixed it.
I also enabled "WMM" (WiFi Multimedia) - If I had known what WMM meant without googling it I would probably have enabled it before.
My router reset itself a while back so WMM was probably enabled previously and I'd overlooked it.