Would this be an original Yuasa battery or one of the other similar makes, if so my Yuasa('s pair of) started to slow up when cold starting at about 8/9 years but never actually failed to start the engine.

You mention replacing the glow plugs, were these genuine or good quality replacements like Beru?, if not then there is a possibility that the glow plugs are failing again, might be worth checking the current being drawn by the plugs....as a very rough guide do you notice the headlights brighten up the moment you hear the glow plug timer relay click off, could be anything up to 2 minutes after starting in the cold.

Have you checked the battery electrolyte levels and tried giving it a trickle charge over the weekend, worth doing periodically durng the winter anyway.

You are trying to start a diesel engine with a 9 year old battery! What do you expect?-just go buy a good quality new battery, how much longer do you expect it to last?