A3 glow plug light on, not flashing, engine won't start, leave for 20 mins and starts

[Dom244]

Hi. My 2007 diesal Audi A3 glow plug light comes on half time when I go out to it and car won't start. If you leave the car for 20 minutes it then starts. I took it to halfords garage who plugged it into diagnostic that said replace battery. They replaced battery free as under warranty. There is still same problem. This started a week ago. What could it be? Garage said glow plugs were ok. Thanks Dominic