Hi. Looking for advice. 
my son has a fiesta and just the other day when filling it up with diesel it would not start after. Right fuel went in so not a mistaken fuel issue. 
got it started by bumping t down road. 
when he went to start it, it’s as if the starter is stuck, just clicks, but not a flat battery! 
once bumped it’s fine and runs ok. Then after it’ll start again no problems, but leave it a few hours and it might not! Had connections look at, thinking bad earth or loose one somewhere, but all ok. Both keys appear fine. 
info says “battery” but main battery at 12+ volts! 
anyone able to help, many that in advance. 

CAnt rule out starter motor fault.  Eg. Worn out carbon brushes or carbon brushes stuck so they don’t slide freely in the holders or the commutator is worn and needs the grooves recut.  Common for these things years ago. Starter motors these days seem to last longer but the parts inside are the same as 50 years ago so in theory same faults can occur.  Tapping them sharply with the end of a wheel brace was often effective to make it work each time.  

Sometimes the fault was the short link cable from the solenoid mounted on side of starter motor to the actual starter motor. A thick heavy duty cable . I had a fiesta 20 years ago where that short link cable corroded right through the middle .

Thanks for the reply’s guys. 
I had the starter replaced. Still the same. Looking at further earths, as apparently one from gearbox can be dodgy. 
other than that there’s a switch behind clutch, starter relay fuse and possibly other things.  It’s perplexing to say the least! 

I know it's no use to you but I had the same trouble with my old vw polo. I could not trust it to start  some times it would, no probs then next time it wouldn't all you got was a click when you tried to start.

Does it still show 'battery' on the dash when it fails to start?

I'm having a similar issue, but mine also bangs and grinds also had the starter and battery replaced with the same issue happening, did you ever find the cause or the solution? Thanks!