I find "mending plates" like these from Screwfix:-

http://www.screwfix.com/prods/16050/...6mm-Pack-of-10

are often useful, also the corner versions of the same. However I
can't find any supplier who has them in anything other than plain or
BZP steel neither of which is particularly good for use outdoors.

Does anyone know where I might find anything similar in stainless
steel or even aluminium or brass or galvanized.

Yes, I know I could make them myself from the piles of odd bits of
stainless and ali I have lying around but off the shelf plates with
ready countersunk holes are very convenient.

--
Chris Green