What information is it reasonable for a freeholder to demand?

For the first time ever (I have been renting out flats since 1984) I have a situation where a freeholder is choosing to enforce the clause in the lease which demands knowledge of the underletting.

Whilst there is no doubt whatsoever that they have a legal (if not, in my opinion, an entirely moral) right to do this I question just how much information they are entitled to demand. Further, is some of it even legal for me to pass on to them under the new GDPR regime?

What they are asking for:
• A copy of the current AST (which contains personal details of my tenant beyond their name – such as their email address and phone number)
• To confirm whether the property is let to asylum seekers, DSS (sic), or students
• Emergency contact details for myself and for the tenant
• To pay various fees

Leaving aside the last point as I already know how I intend to respond on that one, I struggle to understand what legitimate reasons they have for wanting this information beyond using it as a basis on which to pursue further fees from me.

Whilst I can understand that the status of the tenant can affect some insurances (it certainly does have an impact on the standard landlord insurances that I put in place) I have never become aware that it is an issue for the kind of block insurance put in place by the freeholder (more accurately, the management company running the block).

The full text of the relevant clause from the lease is reproduced below.

I guess what I am really interested in understanding is just how little (as a sheer matter of principle on my part) I can get away with providing in order to be strictly in compliance with the lease.

I am tempted to respond with this and nothing more:
• The name of the tenant
• The start date of the tenancy and the initial period (no copy of the AST itself)
• The status of the tenant

The clause from the lease is somewhat vague and open-ended and reads in full:

“To give to the Landlord and the Management Company notice of every dealing with or underletting or transmission of the legal estate in the Demised Premises including all mortgages or legal charges of the Demised Premises within twenty one days after the same shall occur and to pay to each of the Landlord and the Management Company such reasonable registration fees (but being not less that £50.00) and any VAT properly chargeable thereon as the Landlord the Management Company and the Estate Management Company respectively shall from time to time determine.”

I have owned the particular flat concerned since 2006 and this is the first time that any such demand has been made.

Many thanks

Badger