My 2010 Focus 2.0 tdci has developed a fault.

I don't use the car very often which is probably not good for it, but went to start it after about 2 weeks since its last use and all of the lights (internal dash and external) started flashing uncontrollably and could hear a relay clicking. The car wouldn't start so took off the battery and charged it.
Put the battery back on and the car started fine, went back to it within the hour to go somewhere and totally dead.
Charged the battery again yesterday, battery reading 12v, put on car and as soon as the battery was connected I checked the voltage again and was reading less than 3v and dropping rapidly.
I removed every relay next to the battery one by one hoping to find a faulty one but no luck. (I did this as a few months back I did hear a relay randomly clicking while driving which I removed and put back in which solved that problem)
The battery is only just over 3 years old, a superbatt 100.
Any obvious things I need to look for?
Cheers.