igning a tenancy to someone else?

The legal tenant cannot ign the tenancy to you unilaterally. Any tenancy normally requires landlord consent. The legal tenant should check his contract and then broach the subject of igning the tenancy with the landlord. I would be very surprised if the tenant had not breached the terms of the tenancy by subletting to you. The housing ociation could go to court to have you removed but they may look on your case favourably as you have a child.