I also got scammed by a seller on here for £80, a well known seller as there's probably the largest ever thread about it all.
Anyways, I was promised that the money would be returned by cheque on a regular basis. The post office really appeared to be very careless with all the post he was "sending".
So, press ahead, ask your bank how long it takes for a cheque to clear completely and for the funds to not be recalled / bounced (from memory it's actually quite a long time, something into the months). Until this period is up, keep on with the police investigation, and keep records of all calls / emails, including the times and dates etc and pass them on to the cops on a regular basis.
My bet is that the following will unfold:
– the post office have lost the cheque;
– this chap not prepared to send another before the post office investigation time is up (about 28 days or something)
– nothing will ever be sent so it can be tracked
– the cheque never existed in the first place