Yeah the patch applied just fine, and as I said, when I reversed the patch, it still would not compile, so it's just that this particular issue has highlighted a problem.
Weirdly I checked out the sources from CVS...
cvs checkout -P -rOPENBSD_4_2 src
(a) the patch is already there in stable
(b) the CVS sources compile without an error.
Since it's working I'm happy and feel protected
And a lesson learned perhaps... better to use CVS and run stable rather than running release and applying patches.
Cheers
Richard