After writing the above I noticed that the solar panel had been displaced and pointing downwards - the light is mounted on a concrete fence post and the neighbours cat has probably dislodged the panel.
On realigning the panel I noted that the light can also take a external
12V DC power supply and I suspect from the bulkiness of the unit that maybe it has a 12V battery internally rather than the much smaller batteries usually fitted.The solar panel is 20cm x 15cm (behind clear glass) and the light is 80 off individual LEDS.