Don't panic it is NOT an error.
It's just a nag message to tell you it needs a service. If it's not been done in the last year, get it arranged. If it has, you can clear the SE fault yourself by doing the following:
Press and hold the spanner button for about 10 seconds til it glows orange. You should see 1.a in the display.
Now press the Eco button repeatedly til it says 5.a.
When you get 5a press the Chimney Sweep button once (looks like a man in a top hat with a ladder). You will see 00 in the display.
When you have 00, press the "eco" button once only. It will not appear to have any effect but you must do it.
Now press and HOLD the chimney sweep button til you see [] in the display. When you see [], let go of the button.
It will go back to 5.a
Press the spanner just once, and it will go back out of the service mode into normal operation and the "SE" will be gone. Voila.
All thanks to the normal address...
BunSlinger, October 2008