Property Sold but new Tenancy not in place

Tenancy Types and Management

Sold our property which took almost a year. The section 21 Notice I had given tenant had expired a month before completion. Tenant now given 'notice' from new landlord which is invalid as it's just a home made 'eviction notice'. I think the tenant is not ours now as we no longer own the property but Shelter and the local Council say the tenancy 'runs on'. I have told her this is not correct but am I right - I'm beginning to doubt myself? This would mean we are still responsible for maintenance, repairs etc. so doesn't make sense to me. I don't think the purchaser was given the right advice when purchasing the property.