Boo writes:

Thank you for your replies, however, I may have not been clear. The
dogs belong to the person who owns the garden I am working in. All
English back gardens are fenced. I don't think there is a scooping
service in Sussex, UK. The person does not walk the dogs, but just lets
them into the garden, hence the mess. I have done as Dan says, rubber
gloves, and now I wear wellies, to keep my boots and trousers clean.
It is almost not worth the effort, however, as I have plenty of work.

If you feel that way, I suggest you talk to the owner.

Tell them they are the only customer where you have to deal with that.

If you want the extra work, (or money), offer to clean the stuff up
yourself for a price. Otherwise ask them to remove it before you get
there.

Personally, I don't think gardens and dogs work together.
I have a friend with a very large yard and 3 labs. The yard was
beautiful when they moved in. Now it's a wasteland with many holes
three or more feet deep and not a living plant in sight.