Can a freeholder increase ground rents?



New Member
I own a freehold building containing 5 flats. Each flat was sold years ago with a 999 year lease. The lease states that the annual ground rent payable (to the freeholder) per flat is £25 per year. This figure has stayed the same for over 40 years. The lease says nothing about if this ground rent can be increased.

£25 per year per flat seems very low and I would like to increase it gradually over time. Am I able to do this given that the lease says nothing about increasing the ground rent? On the other hand the lease doesn't say anything about NOT being able to increase it either.