I’ve discovered a great service specifically for Leaseholders, they may be able to assist you with legal advice regarding your damp issue. They are called the Leaseholders Advice Service (website: https://www.lease-advice.org/about-us/)
As a first point of call their website contains lots of useful information & if you need any further help they also have a phone advice service where you can discuss your issue with a legally qualified member of staff. I’ve found them really helpful. Good luck!