Exactly what Sue said! I bought the sureflap with timer a few months back and after a few teething problems which customer services helped me to resolve (my cats have biotherm chips which weren't reading properly, so they told me about a hidden custom mode which changes the read frequency of the unit) and now I'm over the moon!
Our issue is that we have a neighbourhood bengal who kept coming in, and we were having to barricade the cat flap with a chair at night to keep our two in and to stop him from getting in. I bought the sureflap and it has revolutionised my life! I can now stay late at work, go out for the evening or go away overnight safe in the knowledge that my cats will be able to be safe in their house and safely shut in at night. I thoroughly recommend it. Your kitty would need to be microchipped for it to work (they do come with a collar tag, but obviously if kitty loses the collar he cant get in) but if he isn't already I'd say it's worth doing in any case.