As an interesting aside, Mode S parameters displayed to ATC (when selected) include selected altitude (that is, what's in the MCP altitude window), but not (despite NATS protestations) selected pressure setting.
So, if the QNH is different to 1013, the controller will know that an aircraft in descent has selected, say, 5000ft - but, crucially, not whether the flight crew have changed from STD to QNH.
Thus is another safeguard removed by inadequate regulation.