My 2yr old border terrier (neutered) has started to "mark" or "sprinkle" in the house and we have no idea why.
He also for no known reason launches himself at the 13 yr old labrador and bites his leg. Fortunately for the border the lab doesn't retaliate. I managed to stop him tonight for the first time by saying no as he leapt off the sofa when the lab came in the room. I do think this is a domination thing but not really sure what to do as I'm not always quick enough to stop him.

Any help appreciated.

This is 100% domination. Your border collie is wanting to take over. Just because he is neutered, doesn't take away the natural instinct of wanting to be in charge. Some puppies, even when neutered at a very young age, act very much like a male dog as an adult.

You're border collie is a very dominant personality. He is getting older and is figuring out what this world is all about. A good way to get this over and done with is to step up as the human and make both dogs understand that YOU are in charge. Obedience, feeding them after you eat dinner (and make it known that you are eating dinner) and making them walk behind you through doors and such are good ways to let both dogs know that you are the queen of the castle.

Another thing... when dogs start having dominance issues, its very hard to step in as the person and just expect it to stop. Even if you are the leader of your pack, every pack leader has a wingman. Pack order is Alpha (being first) and Beta (being second). So.. if you are alpha, there is going to be a struggle for beta. Unfortunately; there isn't much you can do to stop this. In fact, the more you interfere, the longer this dominance struggle is going to last. Always be present when your dogs are together. Try not to play with toys while both dogs are together, it'll start a major fight. Address the more dominant dog first.. feed him first, etc. This is NOT favoring his behaviors, but it is actually recognizing the shift in position within your pack.

Dog leadership is very interesting. If you literally don't follow the book, you can have a lot of problems. If I were you, I'd contact a GOOD dog trainer... one that knows behavior and pack orientation.. which means no Petsmart/Petco dog training. You can also read books or internet articles..

But, like I said, just be present with both your dogs are together. Interfere only if you absolutely need to. The physical part of this struggle might disappear, but the mental/mind games will continue until the other dog backs down or steps up.

Good luck!