My friend and I both had Xantias (N reg.) with the same problem.  He replaced the switch at the bottom of the ignition barrel - problem solved.  I didn't want to spend money so I used a different solution.  Switch on the engine.  Do not switch on any lights, wipers, or any other electrical equipment.  Wait for 1 minute, then switch the blower to the first setting.  It works - and it stays on.  Well, mine does.
                 Colin Harold
                
 April 2008