Thanks for the responses so far.
To try and answer some of the questions raised:-
Given that it will be boxed in, is there any scope for a square one which might make better use of the space and give you both the volume and the insulation?
Only that it seems they are considerably more expensive, but I get your point. I will look into this further tho'.
You can buy a bare store, box it in and fill in your own insulation, which can give very good levels, but is a faff for maintenance.
Hmm, 3 sides of the box are existing and I haven't got a mass of room behind the cylinder, so could easily end up with air gaps if the insulation doesn't drop down properly.
It might have a negative effect on WBS temperature and thus cleanburn leading to sooting etc. though ..
Anyone got any thoughts on this? I suppose with lots of stratification it could take longer for the return temperature to come up.
The space it has got to go in is only 450mmx500mmx1.8m...
Cheers,
SB.