I think you need to understand that this isnt a legal problem
butmmore of a neighbour problem
it looks as tho you housing association certainly think it is
yes both of you should keep to the rules
there isnt a law that says - if you enforce one then you enforce them all
(which if you think about it is sensible - [if you enforce payment of a debt of £3m then you must enforce one for one penny - clearly doesnt make sense]
and yes I had a hedge that was 6' on my side ( coz I trimmed it) and 8' on the other side witha step down in the middle following the border - and he finally trimmed it ( sort of when it went over ten foot and a year later ) - we got on quite well