Susan wrote:

: The food is not actually left "Out", like putting it outside. There is
: an enclosed room looking onto the garden with a cat flap to go in and
: out. Her food is in here. Yes, we do feed her in front of us but she
: seems to like having a bowl of pellets to nibble on during the night.
: Houdini comes inside the house. I think that in this instance, the
: collar should work fine and after Houdini finds that there is nothing
: around for him to eat, hi might go away. Yesterday we blocked up the
: cat flap and kept our cat indoors with litter tray. In the middle of
: the night though, there was a heck of a racket when the invading
: monster tried to get in. He sat and yowled outside our bedroom window
: demanding to be fed. I wish I knew who he belonged to! Thanks for the
: advice though and one way or another, we will sort him out.

That would have been a good opportunity to open the door, let him in
and examine his collar if any, and squirt some water if needed...

Seriously, though, we are assuming he belongs to someone, has home and
food by implication, but is a bully by nature. But do you really know?
What if he is homeless or effectively so (uncaring owners) and hungry,
just from a big breed? At least your description that he "yowled" for
food suggests that rather than an opportunistic bully.

I might investigate him a little more. I would keep the two cats and
their food separated but feed both until I knew just what is going on.

As Kiran says, don't free-feed your cat. That "she likes to nibble" all
night is hardly a good reason and may lead to health problems later.
Train her to have two square meals a day. Remove and discard the food
after 30 minutes. In a few days she will learn to eat what she wants in
30 minutes.