Wooo-hoo. Done and siliconed. Not bad for a girl! Thanks for all the help!
PS - less draughty now as it was just an open hole on the inside and wide brick gid on the outside so a bit less draughty which is a bonus. Cat won't go near the damn thing yet though. Will have to put her dinner the...
Sorry, I know there have been previous posts about catflaps in walls but this is a new twist......I have an airbrick in the outer cavity with a concrete tunnel from the inside to the outside, so in theory all I would have to do is knock out the gridwork within the airbrick and fit a catflap...