On Tue, 29 Sep 2009 10:15:59 -0700 (PDT), jim wrote:

BUT I do remember an inflatable chimney bung that would do what
you want - can;t remember the tradename tho - sorry!

"chimney balloon" will turn up various models...

One of the links is to Money Saving Expert, apart from tales of CO
posioning from fires used with the chimney blocked, there is the use
of a bin bag suffed with loft insulation.

--
Cheers
Dave.