58 minutes ago, ruggers said:

B.C seem to like it boxed in to prevent fire spread from floor to floor through the vented plates or people resting things against it.

I love the parallel universe they live in.  Our twin wall duct through the bedroom above gets barely warm to the touch with the stove going full tilt.  You could sit a box of matches on it and they would not ignite (please don't try that).  The need to keep anything flammable more than 50mm from the twin wall duct is exceedingly cautions.

I think it's to cover a worst case scenario i.e. a fire in the flue generating abnormal temperatures.

20 hours ago, ruggers said:

For future proofing leave at least 70mm clearance between the twin wall and combustibles

Interesting.  What does that allow for?

@G and J maybe future proofing was the wrong term, but a lot of 6" twin wall requires 50mm clearance, so 60 and the odd few 70mm. So try to base it on the maximum and then if you ever have to replace the flue, you have enough clearance for any brand.

13 minutes ago, ruggers said:

@G and J maybe future proofing was the wrong term, but a lot of 6" twin wall requires 50mm clearance, so 60 and the odd few 70mm. So try to base it on the maximum and then if you ever have to replace the flue, you have enough clearance for any brand.

Ah, understood, thank you. 
 

I’m wondering if putting in a 6” insulated flue for a stove that needs 5” is my normal overengineering/overthinking thing (again!). 
 

My thought process is that lots of stoves need 6” so if we dislike the stove and want to swap it (we have done that before) then it will give us more choice for very little cost.  
 

But this is a stove we may hardly ever use.  Such a puzzle. 

I can't comment since I'm not experienced with stoves, all I've done is look at all the regs and drawings associated with the burner I want to install. I had a meeting with an installer who recommended stepping up to 6" in most cases for various reasons, but best to check for yourself. I'm of the same opinion as you, install for 150 not 125 and you're not limited then.