Freeholder requesting payment for sub-letting.

Hi,
I have received a letter from Estates & Management who collect the ground rent for our flat in Twickenham which we let out.
They say that they act for a company called Realgate who, they say, are their clients. Are Realgate the freeholders?

The letter asks for payment if we sub-let and various amounts are asked for up to £330. After replying that I would be sending the letter to a solicitor they then said the cost would only be £75 for a Deed of covenant to cover each tenancy.

Also in their letter, tucked away near the end, is a paragraph saying that "Our client does not permit sub-letting to.....Tenancies to Housing Associations through a private Social Landlord Scheme/Local Authority."

We let this flat to the local council and they find the Tenant. To my knowledge it is not done through a Housing Association or through a private/social landlord scheme. But what are the definitions & are we ok to do this?
If I send in a copy of the Ten/agreement is will show Hounslow council, is this ok?
Can the freeholder really stop me letting to the council?

Lastly can you tell me how to get hold of a copy of the lease?
Thanks,