Loosing coolant from header CDTi

[Busboy]

I am once again loosing coolant from my header, I had this issue earlier in in the year and firstly fitted new O rings to the cap and it didn't cure the issue, I then fitted a new Genuine X part cap and this seemed to solve the issue but now the problem has once again reared its ugly head.

I was told that the tiny return pipe could be blocked so I did clear a small blockage in the elbow where it returns into the neck of the filler, but even though I have done all I think it could be I am now thinking I should change the header as It may even have a small microscopic split somewhere.

Any thoughts?