Bmw 520d 2008 start up problem

[Jake]

Bmw 520d 2008, the car just developed a starting problem it starts fine when left for a day! Then when gets hot struggles to start, but doesn't fire up. Has new alternator, but batter isn't original so maybe batter isn't getting charge. No clue

[Greydog]

Morning Jake 

A diagnostic scan with a BMW capable scanner would tell a lot more ?

However you can carry out some checks under the bonnet open engine off key out and car asleep (light by the gearstick out) check the voltage at the charge points under the bonnet. Anything below 12.5v would indicate a discharged or failing battery. Engine running check the voltage again should be around 14 + v 

Your problem could be a clogged fuel filter, low fuel pressure or as simple as the fuel pump relay sticking so good diagnostics are needed.

Dave

PS. Welcome to the Forum