Primarily Mode S (in the UK) it is used to display the pilots selected level
Not quite right - Primarily in the UK Mode S is used to obtain surveillance data, such as a position measurement, Mode A code and Mode C altitude information - the DAPS such as Selected Altitude are a secondary function. All UK enroute centres are using Mode S derived surveillance data and have been for some time, with well over half the enroute radars now Mode S equipped.
Mode S is a protocol set used for datalink between a ground interrogator and an airborne transponder, not a set of tools at the centre. The new tools at the centres USE Mode S derived data (to display SFL for example), as do the existing display systems display Mode S derived Mode A and Mode C information.
Silverknapper -With regard setting the correct flight number at the transponder - the Mode S requirements for most countries if you are Mode S equipped require that the Aircraft ID is entered correctly for that flight. It is better that the aircraft operators get used to doing this now rather than waiting until ATC stop using Mode A codes and start using the entered flight ID. These are being monitored for correctness across Europe already, with some European countries looking to use it for Flight identification purposes very soon.
RS