A further thought. From s.78 it is possible to depart from levies in proportion to unit entitlements and have some other mechanism so long as the method is ‘fair’ taking various things into account that are set out in that section. So, you could have a levy determination method that is: Everybody contributes in proportion to unit entitlements (as usual) except that the five east wing units only contribute the cost of hedge maintenance.
The alternative method for levy determination needs a special resolution to be adopted. It could be couched in terms of a rule and registered. So, if fewer than a quarter would be opposed, and ideally with a consensus, you could just resolve at a general meeting that it would be fair to do things that way.