Andrew, if you are going to replace the chain and sprockets anyway, you don't seem to have anything to lose by tightening your old chain on your old sprockets rather than buying a second-hand chain and running it with your old sprockets. You currently have a matched chain and sprockets in terms of wear condition. If you remove the slack from the chain, you can keep using it until the small sprocket becomes visibly on the way out (ultimately, the tips of the sprocket teeth will fall off, if you really want to persist to the ultimate stage of deterioration).
Of course, Ken's offer is just about the ultimate in good deals. If you have two matched sets of chain-and-sprockets, you can use the better one and still have a usable one for emergencies.
