There is probably nothing wrong with your battery. Modern cars use a lot of amps, even when standing still, plus, you can't start a modern car, especially a diesel, without a full voltage output (12 volts +), unlike older cars that eventually start with a badly discharged battery. Even then, you could use a starting handle!
During the winter I put my battery on charge for a few hours at the weekend. If you look on here at my book, the 'Companion', under the Know How forum, you will see my under-bonnet battery box mod. Using plugs a sockets from Maplins, it is easily wired into the battery; you can then just plug your battery charger in, without having to remove the battery cover.
It may be useful to make sure everything is switched off. Because I seldom use the car at night, I have permanently extinguished the courtesy interior and boot lights. The handbook shows how to do it, but you just press and hold down the main front roof light switch for a few seconds, until the lights go out. Just reverse the procedure to bring them back on with the door courtesy switches.