Ok... thanks to Paul & Dragan's help and advice I think I have almost got it!
I reworked things to do away with the Y spline and settled on using an X spline with marker data for the window details - I think this will suit my workflow best as it offers more flexibility on window dimensions and configuration. (I tend do deal with a lot of as-built rather than nice new buildings where the new windows can all be the same!).
The transom for pane with top openers was still tricky but I managed to get that by building the two parts separately and then using a compose node to stack them. (I learnt more about pivot alignment here!).
Now I have a couple of issues remaining... For the top opener I had to compose two pieces of geometry together to form the start (and end) piece. When the windo is scaled one of the pieces grows correctly but the other is stuck at is original size leaving holes in my window frame. (The red circles on the image "window 1")
The second problem is adding the handle to the centre of the bottom of the top opener (blue circle). I tried doing this with X evenly & an arithmetic node to find SegmentYRole but it either doesnt appear (blue circle) because the SegmentYRole returns 0 (for undefined), or it works partially (image window 2) but thinks the default segment is actually another bottom. Not sure why it is different on different configurations.
Even zipped the max file is a little too large for uploading to this site but if anyone is interested it can be found here...https://escapedesign.sharefile.com/d-s2978529c240f42f998267a9650010f16
Any tips would be greatly appreciated as it feels like I'm almost there now!
(in case you're wondering about the black squares in the corner of the glass this seems to be an issue with RC mirror messing with the normals in some way... it does'nt happen every time a piece is mirrored and I've solved it by mirroring the piece in Max itself then using that version in the 'broken' corners)