I know this might sound stupid, but I will say it anyway.
I'm not bothered about the size of the current battery, it's the size of the biggest battery it'll take that I'm after. Although it appears now I've had a good look that it is the biggest battery already fitted.
Can't tell the size written on it because of the way it's installed, without removing it, but I've got the battery dimensions. 243mm Long X 175 Wide X 190mm High.
Alternator should be fine, no warning lights, and I do 50 miles a day, but I will check this as like you say getting a battery is pointless if it is this.
The battery is original having got the date code off of it. Jan 2008 manufactured, and the car was a factory built to order and registered in Oct 2008, so I'm surprised it's lasted this long going by the quality of OEM Fiat batterys most people have fitted
Having been on a charge for 9 hours the battery charger was showing 3/4 charge. I've turned the charger off for 10-15 min while measuring the battery and completed a drop test and it seems to be holding charge well. I've popped it back on for now and see what happens tomorrow. All should be good all being well.
I think the Gadget I fitted on Saturday just gone may have caused this with the car having sat for 39 hours (normally no more than 12 at a time), so I'll test that for current draw this weekend if the battery seems fine over the next 24 hours.