Or is it the slave?
I understand the heavy interest in ecu's as they are vital, but I have also heard that they rarely blow, except when drowned of course.
And even then, if you clean them out quickly enough, good chance it'll be fine. I'd say it's more important knowing where the ECU is, and how to get it out quickly and take it apart than it is to carry a spare.
To answer the original question, I believe the TD5 has a crank position sensor, and that would be my vote for the #1 item to bring. Then since it's a TD5, I think a Nanocom would be sufficient instead of a full blown T4/Testbook. I'd also bring an extra fuel injector. That leaves one more, I'm not sure what.