OK, I have just checked my details on the car, my 2.2 diesel has a glow plug controller and 4 glow plugs.
Glow plugs are switched in by the car automatically in very cold conditions to aid starting. I have not had a problem with my car, I can leave it for days and it just starts 1st time. But historically, with older diesels my experience is that if a cars starts when warm but is difficult or will not start when very cold (frosty) then the first place to look is glow plugs. Glow Plugs are basically little electric heaters and they can burn out over time with higher mileage vehicles. Checking them is difficult, best is with a high current amp meter. They are not very expensive, so one option may be just to replace them and then see, the usual process of elimination. You can do a check with a volt meter on their supply but this won't tell you if the glow plug is burnt out or not. The supply will only switch in by the car on very cold startups and then only for a very short time, which I think varies depending on conditions and then may only be for a few seconds. I do have a pdf document with some details, if you PM me I can send them to you. Richy