Hi after removing an old window I realised the internal opening is smaller than the external opening. But the new window fits across both skins and i'm now left with 40mm gaps around the sides and top of the new window on the outside. I'll try and get some pics up
Just buy a new window and put it down to experience, you can buy make up pieces in UPvc but they look poor. Unless your window is very big it will look odd with the thicker frames. You could always sell your small window or see if the manufacturer can help you out by selling it for you for a cut?
You need to buy a bigger window and then fit it from the outside. Then either render the reveal to cover the front edge of the frame or use pvc strips.
Or you can as you suggest bring the reveal in to fit that window however you would need to tooth out some bricks/blocks rather than simply cut slips as it will crack badly and might be too unstable even with mesh in the render.
However then you'll have a decent few bits of render to replace and its hard to patch render seemlessly so may end up with a conspicuous area.