I have a Vauxhall antara and the warning light came on for what I thought was low oil so I refilled and checked dip stick and all was fine. I have now noticed oil leaking from my car and on further inspection the light was an oil change needed. How much damage will I have caused?
If you checked your oil level and discovered it was low, then topped it off correctly, you'll have caused your engine absolutely no damage.
It does seem as though you have other issues with the oil leakage. You'll need to take this to a shop to get the oil leak itself sorted out. Depending on the amount of oil being lost through the leak, you can drive it with the leak as long as you stay on top of the oil level and ensure that you keep your oil reservoir full. Mind you, with the leak, you'll be polluting the environment, staining your driveway/garage floor, and wasting money (with replacement oil), so take it for what it's worth.
You may have over-filled the oil reservoir, which is dangerous but not necessarily guaranteed to cause problems. As long as the excess oil has somewhere to go, in your case leaking out somewhere, You can still drive it to a workshop and have them take a look. Just drive carefully and don't let the revs go too high, i.e. stay under 3000RPM as much as possible and you should be fine.
EDIT: you may have a perished oil seal, aka a sump plug washer. They're supposed to be replaced every 10 000 miles/15000km. They're about the same price as a large chocolate, but they do a very important job.