<<Despite what HD said what really gets ATC concerned and is also NATS' biggest safety risk is vertical infringements by non-transponding or no Mode C traffic >>
Sorry if I did not make myself clear but Warped Factor knows that non-squawking traffic goes up and down through WOD quite often without ATC taking action as it is assumed to be below CAS. However, if one of those aircraft suddenly enters the London Control Zone then ATC is bound to attempt to provide separation from known traffic.