aquachimp wrote:
1)
Can / would a hedge hog use a cat-flap

Can't see why not if it gets used to it

2)
If we allow it to continue on the cat food, will that be bad for it?

Cat and dog food is what is recommended for feeding hedgehogs, but milk is
not good for them at all

3)
If not bad for it, if we put some food out for it (we would like to
keep it and maybe the bribery would help), will it not bother with
slugs and snail anymore?

It it has a good supply of food, it would be more likely to breed, you could
end up with dozens :-)

Tom