What could be the problem?

My wife tried setting off at junction and it just cut out on her Corsa 02 reg 1.2 SXi Tiptronic, F appeared. Took it to Vauxhall. They said the battery was flat, charged it over night and it worked perfectly with no faults appearing on their computer. Drove away and 30mins later brake down again with the same problem. Tried charging battery myself to get it home next day and nothing. F still appears and it is still immobilised. What could be the problem?