Range rover vogue tdi 2005 turning over but not firing up

Hi. I'm new to the site. I bought my range rover may this year soon after the starter motor went and it was replaced. No problems since then. However last week i made a short journey popped into a shop....went to start car it turned but wouldn't fire. I walked home left the car. The next day I came back to the car and it started. Took it straight to garage and it started fine every time. They did diagnostics and checked the car but nothing showed up. No problem found. This morning the car started no problem. Drove it for 2 mins to local shop....came out the shop it wouldn't start. Left the car.....came back to it 2 hours later.....it started. My mechanic has said without spending thousands trying to find a problem there isn't much they can do. They said take it to a dealership to get them to run diagnostics but that alone could cost me nearly £400 my mechanic already ran 4 separate diagnostics. Does anyone have any idea what this could be? Electrics? Fuel problem? I can't afford to spend a fortune trying to find out what's wrong . Any help would be greatly appreciated.​