hi russell
yes its a blocked censer or the pipe to it, look inside at the top for a round thing with a pipe coming off it pull this pipe off and give a good blow down it if you cant then its blocked if you here bubbels thats good it will be hard to blow down it but when its clear you should hear air comeing from the inside the drum refit the pipe and with luck it should be ok then
michael thorpe, May 2005