Subletting my buy to let property

Other

I bought an investment leasehold flat 15 mths ago. A clause in the lease states that I have to obtain permission from the freeholder to sublet. I did not seek that permission and was not chased for it.

Recently the freehold was sold and the managing agents for the new freeholder is insisting that I seek permission which is entirely right, fair and will benefit the property, the tenants and the building. However, they are demanding £150+vat for granting this permission and are saying that they will continue to charge £100 +vat per annum thereafter whilst I continue to sublet. This charge is apparently to administer the verification of ASTs.

I have 2 questions:

1) there is no mention of charging for granting permission to sublet in the lease. Can they do this?

2) if the answer to 1) is yes, can I charge them a fee for handling their paperwork?