Dear Everyone,
Many thanks for all the advice, much appreciated.
I did wonder myself about the alternator output, though I'd have thought that would have resulted in a warning light. Anyway, I left it on a CITEK trickle charge overnight and it showed green on the meter this morning - which was worrying.
On trying to start the thing, however, it just 'clicked' and out went all the lights, so I'm pretty sure it must be the battery.
I'm going to have a try at removing the existing battery this afternoon - thank you all very much for the input - I wasn't too impressed with the nature of the tiny screws holding in either the parcel shelf or the tool shelf, had anyone thought up some more efficient method? Dzus fasteners or similar?