[XC50077]

Thanks Russell,
Had both battery cables off overnight. Cleaned earths earlier today, no difference. Then drove car about 10k just now and light has gone off and resumed it's normal pattern of flash every 2-3 seconds when parked.I had noticed the smartshield light was on but not flashing when parked for a few days earlier this week. Hopefully the problem has solved itself.
BTW the battery is 3.5 years old but seems to charge and hold voltage ok at 12.7 volts tested with my cheap multimeter.