If chain is stretched that much you should be looking at replacing it.I definitely need to do something about the chain. It's so slack it came off this morning. Normally that's a 10-second job to put right, but this bike has a coaster brake, so the normal back pedalling method wouldn't work. It was a bit of a faff to get it back on, but I got there. It really is very slack and needs a link or two removing. BUT I will bear the advice above in mind.