Disco 3 Immobilised & wont start

Hi all,

This is my first Disco & having a host of problems:
Main one at present:
Car started fine last few days. Today all dash light switch on but engine does not turn over ....
Had Green Flag out and they ensured battery was fully charged and tried again but finally gave up !

At one point all the dash began flashing & flickering..Battery also went from 11.5vo;ts to 8 volts

Can somebody help?

Thanks