Does anyone know if you are better off installing a 6″ flexi flue if it’s possible, or are 5″ flues just the same / as good (in terms of best for a good fire, good draw etc). If so what’s the point of 5″ flues?, are they a compromise for difficult dogleg chimneys or something?. I was going to put in a 5″ flexi flue, but after inspecting from the chimney end, we’ve realised it’s pretty much a straight chimney, so not sure what to do now.
Any advice welcome.

There are some Building regs about it, and unless it has been approved for use with a 5″ flue, then you shoudl use a 6″, though there are exceptions.
I had the same thought when fitting mine – the outlet at the back is 5″, but it is better overall to fit the 6″ I learnt.
Loads of info here:
http://www.stovefittersmanual.co.uk/articles/can-i-fit-a-five-inch-chimney-flue-liner/

Go 6″ if at all possible. 5″ works fine, but will get dirty more quicly. Burn clean and this isn’t a problem. I only use 5 if the chimney is too tight for 6. I’ve used 500+M of liner over the past couple of years and only two or three of those chimneys needed 5″.

Having just had my installation signed off with a 5″ liner I suggest you check out the building regs for stoves available from stoves-online.co.uk which is an excellent source of information. Essentially you can only have a 5″ flue if your stove is smaller than 5kw and is DEFRA approved which mine is. Other than this you need a 6″ & if the info I read is to be believed many stove installers won’t fit a stove with a 5″ flue so if you have the room in your chimney for it you’re probably best to go straight in with 6″. I actually wish I’d gone for 6″ as I would like to change my stove for a bigger one but would now have to replace the flue too.
Edit, beaten to it!

Ok thanks, 6″ it is then as it seems sensible to have more flexibility when buying the stove and not be unnecessarily restricted by regs.