I understand that you owe 40 days Knights Service to your Lord but that you can also be summoned above and beyond that to defend your homeland.

So what exactly is your homeland? Your County? Your Duchy? Your Kingdom?

And what if your Earl has you spend your 40 days service doing one thing and then his Duke or King then approaches him and asks that he raise his army for service to the King. is the Earl left looking like a bit of a fool as he turns up on his own?

Thanks in advance for any replies.

The King says it is the Kingdom, The Dukes say it is the Duchy and the Counts say that it is the County. :P

Historically, there was a dispute in England whether or not English barons could be made to fight in France to protect the old Angevin possessions on the continent, as they were not part of the English Kingdom. Note that this dispute was over the 40 days service, not about the defend your homeland! With this in mind, I probably would be inclined to apply a more stringent criteria for the 'homeland' and say that it is the County or, at best, the Duchy. That means that all of those big battles against the Saxons will go under the 40 day campaign rule. (Not that my players' knights mind so much, since campaigning tends to mean better looting opportunities, so they don't mind serving extra.)

Kings generally don't go off into a war without preparing for it first, and they certainly would let the Dukes and the Counts know in advance when to muster and with what forces. So it is unlikely that the Earl would end up 'spent' before the King comes calling. In the hypothetical case that this were to happen, then the Earl would have to make up for the lack somehow, either cajoling his vassals to serve extra days (promises of loot and lordly benevolence vs. pissing your lord off), paying them to serve, or hire actual mercenaries. Historically, though, the Lords often did end up short in the musters, with various excuses, but that could be dangerous for the Lord.

Great answers, thank you.

Also remember that the Earls and Dukes are advisors to the King. If the King goes off to war it is probably with their blessing and maybe even their suggestion to go in the first place. A smart king does not want to look foolish if he goes off to battle with only a few men.