Property sale with existing tenant

Other

We have a flat with a tenant who currently has a Periodic Tenancy. We have been in contact with a property sales agent who advised that an investor is keen to purchase the flat and he would be in agreement to take over the tenancy with the current tenant as she is a very good tenant.
Is the transfer of tenancy generally a straight-forward process or are there certain obstacles to watch out for? Also what type of legal communication to the tenant from the landlord is required to start off the sales/tenancy-transfer process?
I appreciate your advice. Thanks