Thanks Keith. It seems to only be this one device, a Windows laptop.
I think it's possibly a faulty wireless NIC, or something else locally on the laptop. As when it drops, it can't see the WLAN anymore, then when it joins a different WLAN then the original one will appear again and can rejoin it.
We've checked the signal strength and that doesn't seem to be an issue, can't see any interference from other WLANs. Not sure about interference yet from other devices on the signal.