Hello, I am a solicitor with 20 years experience. I will try to help you with this.
The basic position is that for a repair like that, as long as it is actually necessary, you freeholder will serve a notice in advnce about the work and notify you of the amount required for the work. There will be a procedure by which you can obtain information or challenge the sum sought. The leaseholder can not do the work and bill the otherleaseho9lders, only the landlorrd/freeholder can.
Have you received any notices or has the work been started yet?