This seems to be the most common post in Winter

. Whenever you have this situation (starter not working, battery goes flat) I always suggest jump starting the car if possible . With the added voltage from a good battery the starter should turn the engine quickly and the car should start within a second or two. If the starter still churns slowly then it needs replacing - a simple enough task. If nothing happens but the car starts O.K on the handle then it's the solenoid. If the starter works fine when jump started then look to recharging problems (Willie covered this) or a dud battery.
Often it's not appreciated that a flat battery needs recharging properly. The engine needs to rev at more than 1000 rpm. I guess it takes 15-20 minutes driving around to do the job properly. However if you're using lights, heater, wipers and other items then the battery hardly gets any chance to recharge, due to the limited recharging ability of the standard Minor dynamo.
Ideally, to avoid many hassles I would suggest fitting a heavy duty battery and owning a starting handle

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A battery has the hardest job to do in Winter - the low temperature reduces the battery's output , there is more engine resistance to the starter and damp in the electrics causes problems. However there is a lot you can do to avoid unnecessary battery drain. Whilst driving, try to switch off wipers/ lights/ heater whenever possible to allow recharging. If this can't be done on the move, leave the engine running when you stop (at very fast idle) for 30 seconds or more after switching off the electrical items. When starting in the morning shut the door & start the car FIRST before switching on anything which drains the battery. . With a borderline battery it can make all the difference

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