my battery's very weak at the moment, previous owner had it fitted a year before i bought it. gradually got worse, so went to to Halfords to get a new one. But they needed the receipt which i(we) didn't have. Doesn't flash up any errors though.
Think what I'd do is reset the ECU (not sure how to on a GT) disconnect the battery, clean and grease the terminals, test the battery's performance. then see what happens. JTD's are big power consumers, so you do need a minimum starting charge. Apparently my battery was giving half of what it should, hence the slow starting.
EDIT: It was a Halfords battery by the way, I'll be getting a Bosch/Motorcraft one next