After some investigation and a long charge, i've determined the cause of the issue is a combination of factors; slightly tired battery on a car that rarely gets a long drive and is started quite a lot, plus reconditioned alternator of undeterminable manufacture that isn't quite as good as a genuine Ford item, plus having the lights, heater blower, radio, wipers and heated screen on, added with a dash of ridiculously cold weather leads to battery going flat.
It seems that whilst it was on charge it was 'waking up' the battery, and it actually took more amperage as the time went on, instead of going down like usual.
Also i tested all the electrics with a voltmeter on the battery with the engine on, and with everything but the blower on it still showed 13.1V (alternator output), but then when i put the blower on it fell quite quickly to about 11V and continued down slowly.
So after a big charge it should hopefully be ok for a while.