Excessive charges?

Tenancy Deposits

Landlord has charged tenant over half their deposit (i.e. %C2£750 of a %C2£1400 deposit) They have been in property for 3 years with no real issues, bar the odd maintenance request. They've been no trouble, have paid on time every month. The wear and tear they are asking the tenant to pay for is nearly %C2£500 for grubby walls - does this seem excessive - the owner has not done regular maintenance ie. painting annually. As a landlord myself - I am thinking their charges are really high - what would you suggest they do? They are already in new tenanted property elsewhere.