Well I went in to Mercedes Skipton who apparently dont have a body shop so couldnt give an full appraisal, dont know anything about crinkly seams etc. I have to go to Leeds for that...
Went to the approved bodyshop they use for out of warranty repairs, showed them the rust and they told me that it wasnt rust as such although it is a corrosion that looks to have been caused by something reacting from the product line.
It is unlikely to progress at a rate that will be of any significance for many years and if it does it looks like it would trouble the inside skin rather than the outer one that you see day to day. His advice, 1) pursue Mercedes as it is clearly a design fault, so regardless of warranty deals they should take responsibility for it and sort it. 2) ignore it as it unlikely to cause much bother anyway.
I asked him if it was caused by water getting in and he said that the part effected was sealed and putting cavity wax in it wouldnt do any good.