Weird was designed in to the K12/13 from the start...
16 miles is just about the lower limit for putting back what the starter takes out. They always used to reckon on a 20 mile fast run, and that was back in the day before we were all driving round with daytime running lights on, and in the case of the K, with dip beam sucking amps out to boot.
Firstly, what make and type of battery is it?
Second, when you start it at home, does it burst into life or have a few grinds of the starter before it fires?
As Rae says, it may be new battery time
Hi, the battery is a Nitro YTX14-BS 12Ah and is a couple of years old. Starting at home isn't a problem as the battery will have been on the Optimate - it starts immediately