Boiler seems to be running cooler?

We have a Worcester Heatslave 20/25 boiler and it has been fine so far with regular maintenance. when the heating is on the temperature in the kitchen easily reaches 21/22 degrees which is fine, but recently the kitchen has struggled to get up to 19 degrees.

The radiator valve setting hasn't been changed and the main temperature dial on the boiler itself hasn't been changed.

Is it possible that you can override the temperature of the heating system from the inside of the box?

Recent row with boyfriend over heating leads me to suspect foul play and underhand tactics might have been employed to keep the bills down...

OR, it might just need another service - anyone have any ideas or suggestions?