So the rules contain a list of eligible ladies, each of which own a substantial amount of land that would set the player Knights up as pretty rich were they to marry any of them. All my players are trying to win one of these but my problem is figuring out what they would need to do to be allowed to marry one of them.

Well, there are no hard and fast rules here, and a lot depends on were you want to go with your game. If you want your PKs to be rich, important persons at court, and be that soon, you could be generous. If all that power and responsibility puts you off, you could be stingy. The PCG is long, and you may want your players have to work hard for decades to earn more than one manor (by which time these "starting" ladies have been married to other knights long ago, but been replaced, hopefully, with younger ladies with land on the market, so to speak).

You could also consider that a Lord has a lot of bachelor knights, and he will dangle those ladies in front of them to make the work really hard. They expect him to give some of them the best of the land, in due time. A Lord has little incentive to give land to knights that are already landed. They may get ideas. And the bachelors will grumble.

Now, I would require one of the following for a vassal knight to be allowed to marry a woman with one or more manors attached:

1) Truly heroic action in the service of the Lord, as saving the Lords (or his heir's) life in battle, leading the storming of a castle, turning the tide in battle by killing an enemy commander or champion, defeating a powerful monster that has evaded killing or capture by many other knights (possibly for years on end). Something really bloody and brave, in short. Not just fighting and bleeding, thats required, and will win no manors. Above and beyond, that kind of thing.

2) Truly excellent service in court for at least an decade. Again, just being there and speaking your mind is not enough. It has to be a knight who consistently delivers outstanding advice (often of the "outside the box"-type) that leads to successful decisions and good results, that takes on the hardest diplomatic task or other missions and succeeds beyond what an average knight could do. (Note that most players may not be able to do this, it requires a sharp mind and attention to detail, and, of course, and interest in these kind of events during the sessions).

Yes, it is hard. And I think it should be. Most bachelors never win a manor, and most vassals remains vassals with just one manor for their whole life. Gaining the first manor or a second manor is a huge distinction that marks an knight out from his peers.

Exactly what Skarpskytten said.

Also, read this thread, "Heiresses Revised" by Greg Stafford himself:
http://nocturnal-media.com/forum/index.php?topic=2364.0

Kill all suitors until Anarchy Phase and then convince Ellen that you're really the best option. No one can say you weren't devoted!

Kill all suitors until Anarchy Phase and then convince Ellen that you're really the best option. No one can say you weren't devoted!

One of the players have started this approach although its more by coincidence than design.

One of the players have started this approach although its more by coincidence than design.

That's what they all say... PK's are rats.