I've just done this exact thing on my tdi and the orings were around £4 each.
Prise off the clip, clean up the two hoses and fit your new o ring.
Refit the hose and make sure it is 100% fully flush fitted before fitting the clip.
Mine looked like it was on but the new oring being new meant the hose was just slightly not in its home position and popped off after a short drive.
There is a recent thread with the oring part numbers.
Hope this helps.
Jas