I'm involved with installing the new Mode S radars, terminal and en-route not multi-lateration, and we can now see aircraft moving around the airport on Mode S. Unfortunately some aircraft do not disable their transponders when the get to the stand and sit there squawking hour after hour.
I've not seen the situation at LHR but certainly the case at LGW, FRA and DUS.
As there's no response to Mode A/C, RF interference is minimal but 20/30 labels virtually on top of each other tends to clutter the center of the screen! I suspect a gentle reminder to turn off may be forthcoming soon.