as an former fire fighter with a few years of experience of working round chimney stacks ( 22+ to be exact - well sort of ) this FAQ made me chuckle
quote from site : Q. If you install a S.E.T to an existing chimney stack, can you still have a working fire?
A. Yes, the product is heat resistant.
er - heat and soot particles will soon screw this device in my opinion - that's if it does not clog up first

+ soot is insidiously corrosive ,dependent on whats being burned - and if you have a chimney fire ?? - 700+ quid up the Swanee - a half decent chimney fire would probably melt it !! - and believe me i have seen and had to deal with some real "roman candle " jobs

plus agree fully with the thought about NOT mounting things on chimney stacks - from my experience , these are usually the most neglected feature of any house as they are usually inaccessible + i wonder if this firm has any REAL experience of working on roofs/chimneys/at height ?? - roofs are bloody dangerous places for the inexperienced /unwary - and to even THINK of installing one would require proper scaffolding to be erected ( not something you can do off a roof ladder ( imo + there's prob a health and safety issue these days ) - sooooo add a few hundred for that + i wonder if they have thought about someone having an accident following THEIR instructions - or whether your insurance company would approve of a weighty object perched at height with the potential to fall off and possibly kill or injure someone ?? ( just looking at the stacks round here out of my window - i would not want to mount a TV aerial on a lot of them - especially that one over there with a bloody great crack right down it and a nasty lean to the right , there gonna be a big crash one night ..................glad i don't have a car to park anywhere near that property ......

)
its all very well coming up with these things -BUT , one has to consider practicalities / have some experience in the area concerned , like many of the things touted as the next solution , a list of FAQ's on a flash web site , and the real world are 2 totally different things
final thought : what if you DO buy one and the firm gos tits up in a few months ( not unknown ) - where is your back up ?? - repairs for the product, spare parts , servicing etc ?? - not a "fit and forget " item is it ??

definitely buyer beware methinks
