Ford Focus (06 Diesel) wouldn't start, now does

My Ford Focus want starting this morning or yesterday evening – turning over fine just not firing up.

I was about to tow it to a mechanic but this evening, my wife tried it, still wouldn’t work, then put into first gear, tried again, playing with the accelerator and clutch and then the engine coughed a small bit and fired into action! Now it starts fine just turning the key normally!

I should also mention the other day it refused to start, same symptom, but started fine on the second try.

So what was actually wrong here? I’d like to know because I’m not convinced it’s just fixed and am thinking that I’ll have the same problem again but getting worse each time, so I might still have to get something fixed