The air gap above a stove (between top of stove and closure plate or register plate) is not covered in Building regs. However it may be specified in the manufacturer’s fitting instructions so check this prior to buying the stove (it rarely is mentioned… Julian).
As a rough guide I often suggest 35cm minimum (a 35cm gap usually provides enough room for two elbows and a soot door if required or a straight with a door). It is good to have 20-30cm anyway – to allow heat to escape from the recess.
Elbows combined to create an offset can be used to move your stove away from the back wall of a recess (to provide an air gap behind the stove, usually 3″ minimum required by stove manufacturers).
6″ 30 degrees = approx. 4″ offset
6″ 45 degrees = approx. 5″ offset
5″ 30 degrees = approx. 4″ offset
5″ 45 degrees = approx. 5″ offset
For more info on whether you need an offset or not see this page – the Fitting Pack section (this section guides you through what to measure and what to buy).
For detailed flue pipe dimensions and elbow offset dimensions see this page
Why have a soot door in your pipe? Because some stoves cannot be swept “through the stove”. If one can sweep through the stove then a soot door is usually not required.
Note: I have managed to cut down two 30 degree DEKO branded elbows and reduced the offset length down to 16cm (in other words I have managed to create an offset of 3″ in just 16cm height of pipe). I have not yet experimented on two 45’s but I believe I would be able to get down to 20cm with some careful cutting. Google DEKO vitreous flue pipe to see if you can source any.
Behind the stove I usually suggest a minimum 7.5cm but get another inch or two if you can (there’s me mixing my old and new money again).