My pen has returned!
I've just collected the parcel and had a look at it. First, I'm very pleasantly surprised by the fast turn around time. My pen reached MB on the 23rd and I think they sent it out again the very same day. I'm quite happy about that.
Secondly, and unfortunately, I'm not quite sure what exactly was done about the problem. The documents that were included in the parcel mention a tier 2 service and that apparently the cap and the feed were supposed to be examined. Additionally the invoice mentions an item named 'cap'. However, the cap is still the old one and thus was neither replaced nor cleaned, which is what I was hoping for. It is a bit disappointing and I'm going to give MB a ring tomorrow to inquire further and ask whether I should send it in a second time for the cap to be replaced.
I understand that my problem might be a purely cosmetic concern, for the time being at least; but should the corrosion progress further it might lead to a structural failure at some point. And I don't consider it completely out of line to expect a replacement of the part in question, especially in a pen of that price and the minimal amount of use to which it has been subjected so far.
Anyway, I'll see what I can do and keep you updated .