It sounds like the flue just needs some extra support in the loft.
Phone Corgi to get phone numbers of Corgi Engineers in your area.
Or check the diameter of the flue then check your local plumbers merchants.You may be able to find something similar for use with Rainwater downpipes,or try making something to support it yourself.
Do NOT remove any part of the flue pipe.
Gasman
August 2008