Hi Alex,
Do you know how long the squatter has been there? If it has been for a few months or a couple of years then you should be fine to evict them. Sometimes (very rarely), adverse possession claims come up. However a squatter needs to have taken possession and used the property as their own for 12 years. I'm assuming that situation doesn't apply to you.
Therefore, I suggest that you give the police a call and ask them to evict the squatter. They have no legal rights to be there.