Acts as if the turbo has suddenly gone off. A warning 'beep' is heard , then the message 'Anti polution warning' is shown on main display. When engine is switched off then after a while restarted, everything reverts to normal.
Any ideas? ( l know it may be the 'computer control')

Sounds like you have a faulty sensor and the car is going into 'safe mode'. Need to get it connected to an engine managment reader to be sure.

l had a 405 Mi16 and the check engine light would occasionally come on, along with the cooling fan and a noticable loss of acceleration power. In my case it was a short in a wiring harness. Since you said that everything returns to normal after shutting the car off and then waiting and restarting the engine, it is most likely not a mechanical issue. Could be an oxygen sensor, or a problem with the ECM. Take it to your mechanic and have them hook it up to a diagnostic computer.