Annoying Powertrain Light / Glow Plugs Issue

The powertrain light (exclamation mark in a cog) keeps coming on in my '06 1.6 TDCi Focus. When it comes on it also goes into safe mode and drastically reduces the power. It's not dpf related as I had that little nightmare removed and the ecu reprogrammed a while back.

I had it scanned a few weeks ago and the mechanic said it was indicating an error with glow plugs / glow plug module. He reckoned it needed a new module at a cost of €300, or else he suggested there might be a wiring issue somewhere on the glow plug circuit. I left it to have a think about what to do.

The glow plug light comes on as normal when I turn on the ignition. Shortly after starting and driving the car a few miles the powertrain light will come on and I'll lose power. I decided to have a look at the fuses in the fuse box. Anything glow plug related seemed fine, no fuses blown. I pressed all the fuses and relays down just to make sure that none had come loose. After I did this I started and drove the car and it was fine, no powertrain light. It stayed fine, no problems, for a few days, and then eventually the light came back on. Now, whenever the light comes on again, I open the bonnet and press down all the fuses and relays and it's fine again. Sometimes for a few hours, sometimes a few days. But always the powertrain light will eventually come back on again and cut the power.

The fact that it seems to be ok after I've fiddled in the fuse box suggests to me that the problem is not with the glow plug module as the mech suggested. Could I have a duff relay or fuse? Or could a fuse or relay be working loose and causing this?

Any ideas or suggestions would be appreciated!