Can someone explain to me exactly how a Parkray fire works?

I have a 1982 Parkray coal fire and it is quite smokey normally although it does work reasonably well but I have now noticed some smoke is leaking out from under my mantel piece. I will of course ask the advice of a chminey sweep asap (and I did have it swept last summer) but I cannot find any diagrams or instructions for use of Parkrays on the web and I was wondering if someone could explain to me exactly how these fireplaces work so I can undertand it a bit better