A friend of mine is having an intermittent starting problem with their TD5.

Most of the time it starts fine.

Occasionally, it refuses to start, engine just turning with no indication of firing. If left turning for 15+ seconds it gradually comes to life and starts.

This often happens when the engine is hot, which has got us puzzled as it might be more understandable from cold if the engine isn't heating up properly.

Can anyone suggest what might be wrong?

The sealing washers under the injectors would be my guess.

It could be your starter motor interfering with the voltage and driving the ECU insane. Though that normally happens when hot and cold. The only way to check that is to disconnect the starter and power it from another battery.

But I agree with Steve that it could be the injector washers. Pretty easy to do and cheap as well. You will need a small prybar to get the injectors out without damage though.