You can sell this interest at an auction run by a firm called Foster & Cranfield.
Expect to get, at best, the amount you are due at current house prices discounted at 4 to 5% pa (compound) for the life expectancy of the occupier of the house. As an example, if the occupier is a 50 year old female, expect to receive around 15% of the amount you would get if the occupier died tomorrow. You would get a lump sum, which you could invest in an annuity if you wish. F&C can give you a better idea of value than I can.
I assume that you have safeguards to prevent the occupier from remortgaging the house and so on? What are the provisions in the deed dealing with the position if the occupier wants to move house or has to go into a nursing home?
You may prefer to hang on to the interest, as you can see that you won't get a lot for it, but ask F&C for an estimate first, just in case I'm wrong.