Peter,
These can be had cheaper here up-North, if this helps. I saw some in a reclaimed building materials yard near Carnforth for £10-£15, so if you are planning a week-end in the Lakes I can even pick one up for you.
In a "gabled house" enclosure type I think they should go where they are designed to go - on top of the chimney, not replace anything. In an "igloo" version the taller ones will look great - depending on how tall you want your chimney.
One negative side with most (not all) of these is that they will not shield the chimney from rain - quite important in our climate here
Some form of a rain shield and possibly a spark/bird (depending which way you look) arrester arrangement would probably be useful.
Just my thoughts ...
W.