JohnMo, thanks for the reply. Any chance you could scribble a sketch of what you mean? I'm not a builder so having a bit of a struggle with the windows. I'm happy enough to do all the wood working and concrete, but thermal bridging and all that is new to me.
"Your area at the bottom of the window is a big cold bridge. You need to look at a continuous insulation line. So at the bottom I would add a layer of Compacfoam and bring in the meet the floor upstand (would make this at least 50mm thick). This will have enough strength to take the loads and remove the cold bridge. Position in the wall depth will depend on how your walls are insulated the window needs to straddle the insulation."