I will be leaving my Focus alone for around a month in (scottish) winter weather. Ordinarily I would make sure to take the vehicle out at least once a week to prevent the battery going flat in the cold, but obviously if I'm away I can't do that. What should I do to prevent it going flat while it's unattended?
Put it on a trickle charger. This will keep the battery topped up and replace the charge used to keep the alarm etc going.
Mains-powered chargers can be found very cheaply from any motor factors, or from chains such as Maplins and Halfords. If your parking space is too far from a power source for that, you can also get solar powered ones, although with typical Scottish winter weather you might need a fairly decent sized one!
A decent battery should last 4 weeks, at least it does in dutch weather, with temperatures above -5 degrees celsius.
If it doesn't, on older cars, you can simply unplug the + connector from the battery. To be safe, make sure the loose cable doesn't make an electrical connection with other parts of the car.
On newer cars (say after 2005), disconnecting the battery might mess with the electronics, although it shouldn't do real harm.
Keep negative terminal of your battery detached from battery's terminal. You may get the desired result.