Works carried out without Landlords consent?

My partner and I were first time buyers and bought a leasehold flat. We went on to swap the bedroom and the kitchen around (they were right next to each other with only a stud wall between them), and we also moved the stud wall 0.5m.

The managing agents at the time had been notified by nosy neighbours of possible works being carried out, they emailed us to ask and we told them we had moved a stud wall slightly and re-fitted the kitchen/bedroom, we never received a reply.

Now selling, we have found out that we had to get permission from the freeholder for changing any arrangements of the property (we thought this was only for structural work, and in the past we have asked for their permission to change a window and for an extractor fan to be fitted). The managing agents have gone to them and suggested applying for a retrospective license, we are still waiting to hear back.

How much do these cost? I have read on forums that the freeholder can charge you however much they like..

Many thanks

Summer