Thanks for pointing this issue out Andrew.
I think its is something the UK owners really need to watch with the climate we have! I'll put a link in this thread in the Euro section. I'm no stranger to rusty metal having spent years trying to keep it at bay on my old Spider, 911 and VW - it was the one thing I didn't think I would have to deal with on the Crossfire!
I popped the seals on mine just before Christmas and was very relieved to find that all was well, but I do try and keep all the door shuts clean. As a precaution I applied some Waxoyl to the seam for the time being but when the weather improves I will clean it off and apply a bead of seam sealer, which, like you stated, should have been there from the start! It one of those things that you automatically do whenever you have welded in a new panel otherwise the rust just comes back within months - I've just spent this weekend doing all the seams on a VW floor pan/tub.