It might take a while for Patch to get used to the cat and then he/she might not be that bothered about the cats presence and you can just let it do its thing. Failing that, they'll brawl it out and avoid each other. I have always found its best to let them sort it out themselves. And, they will.
We had a cat that started coming in through the bathroom window in a flat years back. I tried everything to get rid of her. Turns out my cat really liked it and they just chilled together for a bit in the flat or in the garden and the other cat would go home when it felt like it. We never fed it or did anything to get to to stay, it just did.