Went for a free battery check at
Gaylords Halfords and the lad couldn't get a reading due to 'interference'. He did tell me that because the little inspection hole thing was black, the battery was probably not in great shape - apparently it should be green. AFAIK it's the original Varta battery that came with the car 5 years ago, so I suppose it's done alright.
He also told me the starter motor was on the way out too.
He did say it was a lovely clean car though which helped!