Poujoulat Therminox twin wall chimney is a G50 twin wall - ie 50mm to combustibles - and is available as TI, which has a stainess outer, or ZI, with an AluZinc outer. AluZinc, whilst not as resistant as stainless, gives better corrosion resistance than galvanising and will readily accept paint finishes. For internal runs, especially where it is to be boxed in, you can cut the chimney costs by opting for ZI and then, probably, switching to TI externally.
There are some clever components like the Airtite roof penetration component which helps achieve the airtight and insulated (non-cold bridge) roof penetration so important for the modern house.
You can find the Poujoulat price list here.
If you need further information about this stove, please phone 01803 712361