Starting Issues

About a month ago, I started to have issues with my starter motor. Now every time I try to start, I can hear the starter motor spinning but the solenoid doesn't seem to be engaging the starter gear with the flywheel (takes about three or four attempts then the car will start).

I took the starter out at the weekend and removed the solenoid to give it a clean. It wasn't really that dirty, just some greasy black spots on it.

Put everything back together and it's working even worse than before. Must have taken me about ten attempts to get started this morning.

I'm still assuming this is an issue with the solenoid so I'll probably order a new one but does anyone have any other ideas what it could be?

It's a 200tdi Defender if that makes any difference.

Have you made sure that the drive pinion gear is clean, dry and flies out when the motor turns?
The best way to test it is by using a couple of bits of wire straight from the battery, but be cautious as it will kick hard on start up!
That will let you know if what is supposed to happen actually does.