Battery/starter motor issue?

[SmilerFTM]

Mondeo estate 2016 2.0 diesel....

The other day when I pulled up and turned the engine off when I tried starting it back up, the car acted as if the battery was dead. After a while of ringing around various people to get some jump leads it randomly started again without me doing anything to it when I tried again after it sounded dead.
Fast forward to today and it did the same, my mate went into a shop. I sat with the engine off and the radio on and when I went to start back up the engine tried to turn over initially but was weak and then I got nothing at all. I had zero power to the windows which were down to put them back up. When I pressed the ignition the dash lit up as normal but the radio screen didn't. When I tried to turn it over the dash went blank and I got nothing.
Jump leads did nothing at all and the issue was the same but I managed to bump start it after trying the jump. Got home and turned everything off and it started first time five or so minutes later.
Any ideas on what could be the issue? Start/stop button faulty? Starter motor issue? Wiring? Battery?

[sherman]

12v battery.
They only last 4-5 years generally.
If still on your OE battery it will be dead.
If you test your batteries output it will be 11v or less.
£200ishshould get you one fully fitted.
The RAC etc will supply and fit a battery on your drive if you cant get it to start at all.

[anyoldcardave]

Especially with a Ford, start with the battery, low voltage has a habit of taking out a big fuse too, obviously not in your case, but can happen.

Best check charging rate too, but it is very often the battery causing all sorts of issues.