UK Landlord Forum / Life

I rented out a 7 bed property 13 years ago on an AST on my own. The verbal agreement was I'd have some friends moving in with me as I did. The AST states a 4 bed property, despite all 7 rooms having door locks & beds/furniture provided (not mentioned on AST, just part furnished).

I stayed in the house for about 8 years with renting some of the other rooms with rooms-only AST's. I moved out 5 years ago & I acted as a sub-landlord renting all rooms with room-only AST's & protecting their deposits accordingly. My AST states not to sublet without written permission and whilst I have verbal permission, the head-landlord has had full knowledge of my sub-landlord activity knowing I live 100 miles away. He continues to be perfectly ok with this & in fact supports the subject as he simply has no interest in the property. The tenants pay me direct to which I declare on my tax return and in turn give the head-landlord his monthly rent.

Looking into things deeper I found out there was never or an existing HMO license on the property and upon advising this to the head landlord he accepted he needs to get this done.
I additionally found out that my deposit is still not protected and there never had been a gas safety or fire alarm maintenance checks done.
The HMO license promise subject went on for over a year until I suggested why don't I do the application for him to get the subject rolling for obvious needs.

The HMO officer has picked up on so many things that it's going to require quite a lot of management to organise tradesmen to get the license.
The head-landlord is expecting that that I manage all works and likely take a fee from his rent + materials, it'll work out to be 3-4 months rent in total.
We get on fine but I've decided I wish to part company with the house as managing these works will be a subject that will distract my main career too much & noting I live 100 miles away.

There's currently 6 room-only AST tenants sharing the house, all paying rent to me, 3 of which are within their first 6 months of their room-only AST agreements. 

My question is do I have obligations to hang on to the house until all tenants (sub-tenants) are past their 6 months agreements (to periodic) to then serve notices?
My research indicated with the head-landlord having prior agreement of the house being sub-tenanted, then I may have a right to hand the property back to the head-landlord as it becomes his obligation to accept the sub-tenants now as his tenants? I'm not sure if this is correct?

I realise that the head-landlord may accept a transfer of tenancies noting he'll make more profit but ahead of that I'm trying to find out my options to give notice before I raise the subject to him?

Thanks in advance.