Legal English Eviction advice

I split up with my partner last september. We bought a flat but decided to both move out in separate ways but keep it as an investment until we sorted things out. Hi neice moved in as an unofficial tenant without legalagreement, although I do have permission to sublet. The property and mortgage is in my name only. My ex decided to leave the country and now his niece has stopped paying me rent but won't move out. This is a tricky situation as there's no agreement and I guess she's not cled as a squatter. How do I legally get her out of the home I own?