Possibly Koefoeds?  

Are they close enough for you to see them in person and sit in them? I'm not sure if I've ever seen a high back chair that doesn't have some type of lower back / lumbar support.
It does have an interesting look.

I would guess not Niels Koefoed. But that is because I would guess they are not danish. Stylistically they don't look danish. They look more like something else with danish influence. The wood is also dark for teak.
Since you are in the UK, I would guess they might be of British manufacture and design. Stylistically that could fit. And the dark wood could very well be Afrormosia, which was a popular wood choice for British made danish styled furniture. It starts out its life nearly the same color as teak, and then darkens to a deep chocolate brown that can be almost black. I wouldn't expect the seller to know they are Afrormosia (if indeed they are); if they say anything they will probably say that are teak or walnut.

Thank you both for your help. After relentless searching, I have actually found 6 of these dining chairs but they are much lighter. The seller has provided some images which I've uploaded and they have the marking 'Made in Denmark'. I am still curious as to who designed them?