525d won't start when warm

My BMW 525 diesel starts ok when cold or fully hot, but sometimes it wil not when it is warm.?
Fuel Seems to be reaching the high pressure injection pump, and the
diagnostic computer can find no fault, even when it won't start. The engine cranks ok and sometimes tries to fire. The problem is intermittent, but my mechanic is uanable to diagnose the fault.
I have changed the glow plugs but this has not helped.
Any suggestions would be most appreciated.
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