Change in tenancy

Tax Planning and Financial

Have a property that has been let to four people for 18 months, have just been informed that one of them is leaving. The remaining three are not sure how long they wish to remain and therefore are not prepared to seek a fourth person. As this property is somewhat difficult to manage as it is too far away we have decided to put it on the market. As the four of them signed the AST and notice has been given by one of them it now means the tenancy is terminated with them all. We are prepared to let the three remain until the property is sold, but feel that there is still a need for some form of written agreement, so both sides are secure,obviously a new AST is inappropriate. Has anyone got a solution to the dilemma? Is a House share agreement the answer and if so whose to use?