Hello, welcome to the website. My name is Tony, I can assist you with this.
You would need to make a claim using a Part 8 Claim Form (here: http://hmctsformfinder.justice.gov.uk/HMCTS/GetForm.do?court_forms_id=424)
The claim is made under the Trusts of Land and Appointment of Trustees Act 1996.
There is more useful information here: http://www.crippslink.com/index.php?option=com_content&view=article&id=718:cohabitation-a-review-of-the-trusts-of-land-and-appointment-of-trustees-act-1997&catid=32:property-disputes&Itemid=537