Hi and welcome to the forum.
I had mice in my flat a few months ago too. I live in London and there is no getting away from them here, it's just a case of not encouraging them to choose your flat over the neighboors'.
I was told that they can pass on diseases to piggies but I was also worried about anything they could pass on to us (they pee everywhere on surfaces etc...).
Also be aware that there is never just one mouse, they live in groups and reproduce very quickly.
Are your guineas on the floor? Mine were and I found that I could not get rid of the mice as they were having a field day with all of the food and hay a their disposal. If I sat quietly with the lights off at night I could actually see them running in and out of the piggies cage for food!
In the end I ended up putting the cage on a table with smooth legs and away from the wall, that way they find it really hard to get up. I also used a trap (sorry this might upset some people but they had to go. I used an electric trap which is a lot less cruel than poison, glue or common traps) for a few weeks but to be honest taking the guineas off the floor did the trick. When they do not get access to food they soon go and set up camp somewhere else!
Good luck with it!
Laure