Aqualisa Power Shower has lost Power & Heat?

My Aqualisa Power Shower (Type 3 TE) has lost power. The plumber who installed it said there was an airlock which he cleared but it still didn't improve the situation. When I called him out again, he said it was the cold weather. The weather has been colder in the past without the shower giving any problems. Does anyone have any ideas? There is more than enough hot water in the tank and the house was built in 2003.

Donall Quinn
January 2008