My only comment might be that monument is grey and a nice strong grey but Lexicon is blue based, so I'd look for a gray based white or builders white which apparently has very small touch of black in it albeit it is still white. I think this would keep the undertone matched. For instance, I love green and although I probably won't paint the interior of my home green, Whisper White has a slight green undertone, so even though it's still white, it has the colour tone that I like.

Love the black/charcoal look as accent colour (with brown undertone to complement bricks). if you could sand back some of the wood and stain it, maybe paint some of it black/charcoal to break it up would look amazing. If the wood does not stand up to sanding (no grain or not made for raw wood look) then you could add some panels to break it up/add texture not too much black - weathertex is a material I've used recently and can highly recommend.

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.

Making a feature of that front window would look fabulous, and planting up that low retaining wall with structural plants and throughout the garden, lovely house and setting.