Many thanks for all your suggestions & rummaging - I think I've found a couple of replacement catches.
To our (well, mine at least) British eyes, this is an unusual design but Stateside it is obviously more common & whilst I can't find an exact Yale version
I have found this IVES version in aluminium (or nickel-plated aluminium)
IVES 066 with mortice striker plate from American Locksets for $3.78
Thanks again for all your help. Chris