Freeholder charges

Other

The freeholder of the block of flats where I have two units has recently changed.
The lease specifies that written permission must be obtained from the freeholder for sub-letting purposes. The previous freeholder used to charge a fee each time a new tennant was installed as an administration charge for updating their records.(a totally unreasonable fee,I might add).
The new freeholder has written to me stating that I now need a licence to sub-let which lasts for one year only and then has to be renewed. I have been sent a licence agreement to sign but am reluctant to do so as it would seem that it would leave me in a more unfavourable position. The lease does not mention a licence merely written permission as stated above.
Do you think that they have any legal right to make me sign the agreement or can I write back and say that this is not part of the lease?