Sprint GT Starting Problem

Took Sprint GT (2016) out yesterday. All fine, except stopped at our destination and shop was shut, so after turning off, almost immediately had to re-start. It struggled to turn-over, sounding like a dead-battery. However, it is always on a trickle charge at home, and usually is back up to full charge fairly shortly. Eventually, after leaving it a few mins, it started on the button and spun up like normal. Stopped again on way home and no probs.

Any ideas? I had an Aprilia Falco who used to do this all the time, and I put it down to a poor design which would struggle to turn the pistons when they were both just before compression. However, I don't expect it on a modern bike. Only takes once to set you off worrying everytime you go out.

Any ideas anyone? I will do the obvious and check voltage of battery under starting load. I suppose the battery could be on the way out. The guy before kept it in a shed and not sure he had an optimiser.