Wow loads of great replies in such a short space of time guys.
As soon as I got raised on that river I very quickly went to go all in. Then I stopped for a split second and went to call instead. Then I suddenly realised hang on a sec, think about this. (NL tables were extremely new on
hills at this time, I think we were on 60second timers

).
I went back and thought through various hands he could have. He raised preflop when he was clearly going to get called by a few limpers, this probably means AQ/AK or a big pair. The flop came down KJ2 rainbow. AK or AA are possibilities for him preflop. However he bets these hands on the flop, almost without question. So I discounted these. On the turn the K pairs and he checks again. I think here he PROBABLY bets QQ (not sure, he might still check to play safe) he still checks TT tho. If he had AQ then if he was going to try and steal it it would have been on the flop (although unlikely as there was 3 of us in), but he probably would have checked. However he could have flopped either a set of kings or jacks on the flop. This is very much the sort of thing he would check as well, hoping someone catches up with him. By now on the turn he would either have jacks full or 4 kings. The four kings is a lot less likely for obvious reasons, but it is a definate, slim, possibility. On the river I fill up and lead at the pot. He raises. If we go back though his hand range, what would he do it with? Yes he would raise with AK but we can discount that as he would have bet the flop with it. He folds TT probably pretty much all the time, perhaps he calls if he is a total idiot. With QQ he certainly does not raise. He probably calls but he does not raise thats for sure. If he was going to steal with AQ it would have been by trying a bet on the flop and not by raising on the river. Therefore the only hands he can possibly have are, the most likely, Jacks full with JJ or the slim chance of four kings. This took me the full 60seconds to comprehend and I really couldnt believe I was about to fold a full house using both my hole cards. It was the first time I had ever made such a fold and let me tell you it was AGONISING at the time. After I "tanked" for the full period I typed what I was folding into the chat box. Such was my agony at making this fold, however correct I had worked it out to be, I knew I was going to have nightmares about it forever more

. I folded and he showed. Jacks full. I almost lept out my chair in delight lol. It was a real revelation at the time to me and it opened a whole dimension of poker to me.
You all gave excellent answers here, I am impressed. I sure had much more of a struggle coming to terms with all this when I was actually playing this hand than many of you obviously did. Dont be disheartened if you got it wrong, there is not a single poker player in the world who can get it right even 80% of the time. Im going to continue this thread and post some more hands up if you lot think its a good idea.
Jez.
P.S Some of you mentioned AJ as a possible hand. I have to admit at the time my thought process did not include this hand in his range, I only thought about what I typed above at the time. However now you say this I agree it is reasonable to think he could have AJ at some points during this hand. However he would not raise on the river with it, he would just call (and it probably bets the turn also)