Rent to Rent lease termination?

I signed a 5 year rent to Rent lease for a large property consisting of several flats. I decided to terminate the rent to Rent agreement and gave one years notice, even though the contract was silent on termination, to expire on the contract end date.

The sub-landlord failed to serve notice on time for one tenant, and granted a new tenancy for another tenant to expire after the contact end date. I have agreed terms with a new sub-landlord at a much higher rate, But can’t let 2 flats as they are still occupied.

Until the old tenants go, what rent can I charge the sub-landlord (Rent to Rent landlord). I would argue the new rent is at market rates, and therefore the old landlord should pay this. He wants to pay old rent which was massively discounted, and agreed 7 years ago at a fixed rate for the term.

This is looking like it will go to court, I think market rates as proven by new landlord is fair, but is there anything in law to back this or goes against this?

Many thanks

Michael