Thanks for your reply and apologies for the delay in responding.
Well. as you have an Assured shorthold tenancy, only the parties who are stated as being Tenants are entitled to occupy the property, unless you have given specific consent to them to allow other family members to occupy also, which you didn't.
If the original term of the Tenancy has expired, the Tenancy continues on a "periodic basis" until either you or the Tenants serve Notice to end the Tenancy. As Landlord, you are entitled to give your Tenants 2 months Notice to end the Tenancy, such Notice to expire the day before the monthly rent is due.
Therefore, from what you have told me, they don't have any legal right to allow the husband to move in.
Likewise,even though you intimated to them that they could stay in the proeprty for the rest of their lives, legally you need only give them 2 months notice if you ever did want them to move out.
I hope this gives you some comfort, and sets out the legal position.