Having assembled a complete window yesterday, it became apparent that the frame was approximately 5mm too wide between the side channels for the droplight.   The cause of this was my negkecting to establish all the critical dimensions from the outset and using an extracted frame as a reference.  The problem was relatively easily fixed by cutting the top horizontal roughly in the middle, drawing it together with a ratchet strap and rewelding.  So small was the error that caused this that the angles at the top corner appear unchanged as a result! The top seal remains to be modified and refitted. The rest of the afternoon was spent recovering fittings from the remaining original frames, one left to do.