VW polo 1.9sdi, glow plug light on, car loses power and stalls?

I have a vw polo 1.9sdi r reg, 93,000 miles. Last week my car stalled. It eventually started again after waiting about 10 mins, but sounded dreadful. As soon as i slowed down to turn a corner the glow plug light came on and I lost all power. My mechanic said fuel getting through pump ok, has to be an electrical problem. If car starts and glow plug comes on as normal it sounds ok, but if it starts but glow plug doesnt appear it sounds really bad. Can drive it, but then it stalls and I get towed to the garage again and we are back to the beginning.
Please can anyone offer any advice - as after trying everything he can think of, my mechanic has now run out of ideas. Thank you