Altering an existing AST to include a daughter who reaches the age of 18

Tax Planning and Financial

We have an existing AST (going back 11 months and now a periodic) with a mother and her adult son (who are presently joint tenants under the agreement). The daughter reaches 18 in May and wants to continue living in the property.

We would appreciate advice as to how to draw up the appropriate AST. Will this constitute an entirely new tenancy for all parties or will it be a continuation of the existing one?

Additionally, the tenant owes us £568 in back rent and balance of first months rent. We have written to her and she has agreed to pay this sum by instalments but has not done so. She is paying her rent regularly and we are prepared to allow this situation to continue for the time being as opposed to giving notice with all the various void costs; however we would like this debt to be kept alive into the new tenancy. Any help would be gratefully received.