I like those bricks. Out of the two I’d go Woodland Grey and a textured cladding for the extension.

I'd be worried that Woodland Grey will look too similar to the bricks , so my gut is to go with Monument windows for the contrast , but the problem for me , is that without seeing the overall design and the new extension etc , its hard to make too specific a suggestion .

Yes it would be handy to know the context of the bricks in your build. I like white window frames with these bricks. Monument would also look good. Then you could have either white or black Scyon cladding in any of the available designs.

Thank you for your comments everyone, much appreciated. I am leaning towards Monument for the windows. The house is a late 70's build and the extension is at the back of the house. The extension is really just filling in the outdoor area to make it an all season room. We will be cladding the west side and then have double glazed windows/sliding doors on the north and east side. I like the look of the Axon cladding. We will use the same cladding for the front of the house because we are also changing our front entrance to enable us to turn our study into a 4th bedroom.

Monument for me, Woodland grey is a bit green, Wide vertical cladding if you are wanting a modern look