Bantam 2008 rocam 1.3i
I have a 2008 Bantam rocam 1.3i
The battery is being discharged when switched off and stands overnight
The battery is new and when fully charged retains charge if positive terminal
There's something sticking closed or an electrical short. It's called a parasitic drain and you just have to find it. Check circuits at the fused to see which system is on, then follow the wiring.
Get an Auto electrician to check for current drain in the electrical system. If your vehicle has a Tracking system or "Tracker", This will drain about three amps. Check that the lights and the interior lights are off for over night when the vehicle is switched off and locked overnight.
Even during "Lock Down" you vehicle should be started every or every second day and be allowed to run for a few minutes. It might be a new battery, but if is allowed to stand idle for a few weeks it will start to run down and allow the battery cells to sulphate up.