RubberBiker wrote:

First off, go easy on it. You don't want to damage the smooth inner
layer of the flexi.

Yes, that's was one of my worries.

(solid fuel flexis have a corrugated outer layer, and a smooth inner,
with the edges of the helical inner winding facing downwards - the
idea being to reduce places for moisture/tar to get trapped and start
attacking the stainless)

I use conventional rods & brush - but the angles aren't too tight it
mine. You might like to look at hiring the kit, using a professional
sweep, dragging a brush through on a rope from the top - or simply
caving in to the cost of good rods.

Dragging a brush through from the top sounds feasible, although I
wouldn't like to be trying it when I'm a bit older.

Haven't thought about getting a quote from a sweep. Worth seeing what
they charge, at least.

By the way, I discovered in my abortive attempt to do the job this
morning that the first metre and a half is absolutely clean. Do you
think this might mean the whole thing is absolutely clean? Does soot
tend to only gather higher up? (I only burn wood.)

Eddy.