ok I've disabled WPS although I guess that only helps to prevent new probems rather than fixing the one I've got. Looking at the router wifi logs, there were only two devices connected: my windows phone and my samsung galaxy 3 android tablet. When I switched off the tablet, the weird phone thing on the network stopped appearing / dissappearing. Switching the tablet back on and it's still not appeared again.
Does it sound plausible that my tablet was hacked / was the source of the rogue phone on my network? If so, how can I secure my tabby in the future? I'll reset it obviously but what then?