I have two sets of wooden french windows in one room that I’m trying to draught-proof as much as possible. One thing I have noticed is that even when the door is closed/locked the top and bottom of the doors can still move in and out a bit due to the flexibility of the frame I expect. This leads to draughts.

Is there a latch of some kind out there that that will ‘pull’ the top/bottom of the door into the frame thus improving the seal?

Mortice rack bolts with thumb turn are your feind also check why doors are not closing properly sometimes due to foam draught excluder or lots of paint

http://www.screwfix.com/p/flush-door-bolt-polished-chrome-150mm/58212

They do close fine but with a gap down the middle where the two doors meet – so I put some P strip on it to seal the gap and the door stil closes OK but, without being mechanically held into the frame, the top and bottom are flexing out. If I can fit something to ‘pull them in I’ll have a good seal.

They do already have mortice rack bolts fitted but we don’t use them as I they’ve all broken!
The only issue with them is that the frame holes no longer line up with the bolts unless someone pushes the door in from the outside while someone else turns the key from the inside. That may be my only solution though.

Force them shut and leave them for the winter. Over time the doors will conform to the frame.

(Doors left open are my personal bugbear as I cut and fitted then 3 x varnished every damned one of the 13 internal doors in our house. Keeping them latched shut helps to prevent warping in the long term.)

I think you’re right – we really only use them in nice weather anyway when the draughts don’t matter so much.