I don't think the noise issue is so with much take-offs as these predominantly affect a smaller area in the immediate vicinity of the airport as the aircraft quickly reach higher altitudes and can also execute noise abatement procedures on departure routes. Approaches are different as these expose larger areas to noise from lower altitudes. My understanding is that the Avros have to operate with a lot of thrust on approach as a result of the required flap settings and the steep approach and are therefore the main concern. To be honest, I doubt that the planning permission got the db-figures wrong.
PS. Forgive my ignorance - what is Condition 18?
It is one of the conditions subject to which the planning permission for the LCY expansion was granted in 2016 by the DoT. The Avros are condititon 15, condition 18 stipulates an "Aircraft Noise Categorisation Scheme".