Like most people have said, you need to be consistent - it is no use letting the dog on the furniture and then suddenly saying No you can't get up - either they can or they can't ... visitors notwithstanding ...
With a bigger dog, like a Lab, it is easier, since they can sit on the floor and you can still reach their heads and ears to pet them and make them feel safe, but with Cockers, especially when they are little, you just want to cuddle them I have found ... Also children and Partners undo all your training when they come home and want to hug the little critters ....
It is not un-do-able - you CAN reinforce the STAY DOWN if you all do it all of the time - but I think b***** it - if your visitors don't want a hairy little person being loving, then they probably don't like kids either and maybe they pull the legs off spiders as well ....!
I think this thread probably leads back to the 'Do they sleep on your bed' but as long as they don't get possessive over either the settee or bed, then I think they are being the perfect Cocker really ! As long as they have either an alpha male/female to let them know the rules, then most dogs are happy to go along with what we want them to do