In reply to Andrew W:
A product called Plus Gas is a good release lubricant, I have used on both old Triumph sports cars and old (and not so old) landrovers, plus a bit of use on modern cars for those stubborn bolts, many times better than 3 in 1, WD40 (which really is not a penetrating oil!!).
Another couple of tips, if you get the crank off, put it on a piece of wood and give the pedal a whack, and also try tightening a fraction first, then loosen.
What happens over time is the threads almost sieze together, and possibly also a touch of rust. What you need to do is break this join, which a well judged tap can do. For some reason (I did know, but have forgotten) it is easier to break this join by tightening a touch first, it has to do with how the threads interact I think. Once broken, it should go, unless the threads are really disformed. I would also recommend running a tap through the crank threads before fitting a new pedal to clean em up.