crank has play
 

I have an older Falcon English bike and the crank has about a 1/2 play in it. Does it simply need to be tightened or is this a sign the bottom bracket needs to be replaced?

It probably just needs to be tightened, but i'd take it apart and look at the races and bearings just to be sure. check the lock ring and cup (non drive side) and see if they are moveable by hand. Then this is definitely a "just came loose" kind of thing.

HOWEVER, depending on how far you rode it while loose, you may have done dameage, which is why it's a good idea to take it apart and look at the races to see if they appear out of shape, and the bearing to see if there are any crack/chipped or flattened sides (however if they were you'd probably feel this while riding).

With a 1/2" play, I would expect the worse and hope for the best. IF the adjustable cup is still "locked", I'd REALLY expect the worst!