I am no expert. But my research suggests that the limitation period for recovering a service charge in the case of a Camden lease is 12 years. As the leases are made by deed the service charge falls under section 8 of the Limitation Act 1980.
I note that a Camden lease does not reserve service charges as ‘rent’ which is why section 19 (6 years) does not apply.
Of course, Camden can seek to recover no greater amount than it has notified under the 18 months rule.