If you don't have a battery charger it might be worth getting one so you can at least get your battery charged up. I have a multi-meter so I have in the past used it to see if there is a drain on the battery when the car is stationary. Set up the meter to measure Amps, disconnect the earth lead to the battery -ve terminal, and put the meter leads between the battery terminal and the earth lead. To lock the car you will have to jam down the microswitch that detects that the bonnet is open, otherwise it could trigger the alarm.
When the car is locked and interior lights have all turned off, I wouldn't expect to see a current drain of more than a few tens of milliamps. I may go out later and check what it is on my car.
A drain current of 50mA would take 8.4Ah out of the battery after 1 week, which is fairly small compared to a typical battery capacity of 60Ah. If the current is 500mA, it will flatten the battery in 5 days.
If the alternator is working ok, it should re-charge the battery after an hour of driving.
I would check if you have a drain first, maybe an interior light that isn't turning off, like in the glove box or the boot.
If the battery was from Halfords double-check the battery is definitely the correct one for your car.
