I have had starting problems recently, and i have ordered new glow plugs in the hope that they will resolve the slow starting.
Could you have a fuel supply problem? You could be loosing fuel pressure at the injectors over time, hence the longer you leave it, the worse it gets. If this is the case you are looking at new/reconditioned injector pumps but would possible rule out the fuel pump/filter first!
The "clicking like crazy" occurs when the battery is running out of juice. I have seen many garages run a free winter safety check where they will check if you alternator is charging properly. May be worth checking as if the battery is not topped up fully you will get slow cranking.
I hope to get my glow plugs before this weekend, so it will be interesting to see what difference they make.
jonathan