Glow Plug light on and no power?

Hi all, I have an 04 plate Renault Trafic, 150k and just had the Aircon recharged. Now iv driven it away, the glowplug light comes on when you rev it and I have no power. If the engine runs on idol, the glowplug light goes out. Iv checked around for any wires that arnt working and the aircon place did a diagnostic check for anything electrical not working and all clear. its a friday night and no chance of fixing it over the weekend in a garage, so any ideas?