Saxo 1.1 MK 2 radiator

[afn33]

Give me a clue please because I am absolutely stymied with this damn query that no one is giving me help with, even Citroen do not seem to know what is fitted to their own cars..My daughter has had loads of problems with this Saxo leaking coolant culminating in a garage fitting a brand new rad, problem is the leak is still there and this time its coming from a 5mm hole high up in the filler neck on left hand side when viewing car from the front..

Can anyone tell me please if this very small hole is purposely designed and has a use, such as relieving pressure or has someone been naughty prior to fitting [maybe in the manufacture of this rad] or indeed afterward at this garage and drilled it through, that is for some reason that I do not know of, but it all seems odd to me, very odd indeed..