To those crews who fly into any of the London airports.... just play the game since with Mode S providing IAS data to the guys and girls who are controlling you it is not possible to bull5hit anymore!
Is it selected speed or actual speed readout? With Vertical speed selected you can set the speed at 210 in the MCP but still be doing 300 kts with a V/S of 3000 FPM for example. Not saying I would do this , just saying Mode S might not be foolproof.