Martyn, not sure what you mean by a gap? Im sitting right in front of a 20gal Zilla critter cage and there's not a gap to be seen, except of course the wire mesh on top, but if you have a snake that can wiggle through that, I'm afraid you don't have a snake but a thread. It is US-based, I don't know if they're available in the UK, bitsy or someone would have to help you with that. At a chain pet store like Petco, the critter cages are about $90USD for the 20gal, but I bought mine from an independent petstore for $70USD.