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 so having a transponder on with Mode C is the best thing that anyone can do to mitigate against the effects of an inadvertent infringement of controlled airspace. It ensure all the electronic safety nets, air and ground, are given the best chance of working to maximum effect.
3,000ft in the 2,500ft bit to the west of the zone is an issue, especially so if LHR are on easterlies. The rules say that if we know someone is infringing we have to take 5nm lateral and/or 5,000ft vertical separation from the unknown traffic. In the area described a 3,000ft infringement will disrupt LHR inbound traffic on easterlies... you don't know what runway was in use at LHR at the time?