Not sure what your catch is like, but mine was a stainless steel verion of the old "screw-down" type of sash window catch. It tightened/pulled the hatch down onto a rubber seal.
Most chandlers seem to keep them, I think I actually went really cheap when I replaced mine (it is down below after all) and got a nice solid brass one from a DIY shop in Slough for about £3 or so .....