Finally I get to the last installment of my advice to you! This time, I'll deal with roaming cats.
As I recall, there are several toms who have the nerve to visit YOUR property. And this can be of concern, since you like to attend to your toilet needs outdoors via your own purrsonal cat door.
I've offered advice about roamers before, particularly about discouraging them (see, for example, my entry, The Invading Stranger). But that advice applies largely to North American situations where the cat in question is an indoor-only cat and is bothered by the visual or olfactory presence of another of his species.
In your case, the very methods I would suggest could backfire, that is, they could end up being used against you as well and thus make the use of the yard quite distressing for you. You wouldn't want to wander about and suddenly be sprayed with water or come to a favourite area and find it sprinkled with awful citrus smells, for example.
However, I do have a few suggestions for your consideration.
The first is an electronic cat door ( click here for an American site or try one of these for the United Kingdom Cat Flaps or Infra-red Cat Doors ). If there is a strong possibility that one of the visitors might very well try to enter your home, then you best lock the door. An electronic cat door can do this. Keyed to your collar, it allows you to open and close the door by your very presence near it. But others, who lack the key, cannot. As long as you don't decide to play host to a party of cats by standing near the door while they let themselves in, all would work well.
The second is a cat enclosure, a dedicated, safe space for your purrsonal use that cannot be broached by others. Such things are ideally directly linked to a cat door or cat window of your home. An enclosure would be ideal for your favourite outdoor lounging area (inclusive, of course, of your toilet). You would, of course, still see other cats but they would not be able to get at you. If interested, consult my entry: An Outdoor Room for Cats. And if Themselves and you wished for more outdoor experience, that could be provided in other ways.
Sorry that I could not be more helpful in this regard.
I do wish you the best,