cat mate super selective microchip cat flap with timer £66.54 @ petsgodirect.

This is a bargain to keep unwanted strays out your house, RRP is almost double at £120 uses the cats implanted microchip to allow entry. you can also set up a single program of times that the flap won't allow a particular action, on mine I set it up so it'll only let our cats in at night, not out.

Cat Mate Elite Super Selective cat flap is ideal for any cat and any home Entry operated by your cats’ unique implanted microchip or by the Cat Mate Elite I.D. Disc (not included) for up to 9 cats - keeps all other animals out.LCD Display shows selected entry and exit lock status, cat location and time since flap last used for up to 3 selected cats.

Low battery indicator.
Requires 4 x AA batteries (not included) which will give approximately 12 months operation.
Timer function controls access for all your cats, e.g. to keep cats in at night.
Self-lining up to 100mm (4") thick - suitable for all single / double glazing and any panel 0 - 100mm (0 - 4") thick including metal.
Wall Liner available for wall installation (not included).
Fully draft and weatherproof brush sealed flap
4 way lock
Keeps strays out
Cat location indicator
Timer function
Mounts in walls or glass