Is it always the case that the door opens from the inside using the handle? If it is, it might be that the wire connector for the outside handle has come loose or isn't seated properly anymore.
I don't think you need to take the door skin off for this, although I might be wrong, but you will need a triple square socket - you might have one already, but they're easy to get hold of on eBay - an M8 one. Mine is M8 x 55mm, so there's a little bit of length on it.
There's a plastic cover on the inside of the driver's door, you can see it in this photo:
Take this cover off, and that's where you'll find the triple square screw. Back it out a little - don't take it out completely - and that will enable you to slide out the lock from the handle. You might need to lever it out a little if it's not been taken out before - be careful not to damage the paint. What you should see when you get it out is on the side of the handle, some grooves, and a wire with a tab on the end that should be slotted in place.
If the
problem is what I'm thinking it is, you'll just need to push that tab back into place and the door should open as normal. If it's not attached, or the wire seems damaged, you'll probably need a new locking mechanism, as I don't think you can buy the wire bit separately.
Hope this helps - most mk1 owners have been up close and personal with their doors at some point, so you should have plenty of other
help if this isn't what's wrong with yours...!