If you have time and access to mains power, try a CTEK charger (a suitable model is 3600); this will save a completely discharged battery. Why has it gone flat? Mine can sit for months with just the immobiliser flashing - perhaps you have a clock or some other device like an extra alarm that is draining the battery? As advised above, cheap jump leads often don't work, and if you do use good leads, leave connected for 5-10 mins with "donor" engine running before trying to start.
The brakes will free off - often with a "bang" - when you move the car. Next time, park with the gearbox in gear, not with the handbrake.
If all fails, join the AA, wait a few days, and get them to recover you to the dealer!
