Random fail to start

Today my 2019 2.0lt diesel L&K had a random failed start and I'm keen to attempt avoid an other so would appreciate any pointers.

I have had the car for 4 months and it hasn't missed a beat. Today was a chilly morning around 2C. Car started instantly and we drove 1.5m to a park after using the heated windscreens and seats. We were at the park for 20-30 mins before returning to the car. Once in, clutch depressed, pressed start button, and the car crank, crank, crank, but failed to start. Turned off ignition, and repeated to have the same result. Tried this four or five times with no luck. This was totally out of the blue and I had no idea what else to try. Being close to home we walked back with the plan to call breakdown later that day. 

Around two hours later I was about to call breakdown but decided to walk back to the car and give it another go (temp now around 6C). Car fired up instantly without any bother, no labouring or cranking! 

Later that day I took the car round to my local Halfords (car shop) who did a free battery health check. I watched his machine which reported 87% charge. He said it was a bit low, but nothing that a good drive wouldn't rectify. 

We do mostly short journeys, and with this colder weather are heavy on all the comforts like heated windscreen / seats. The car has never displayed a low battery warning, and the stop start works all the time (unfortunately ).

So, any idea what may have been the cause? Battery or maybe something else? Any tests I can do to better understand what happened? 

Thanks