IS yours the later one with the 13mm head and the large flange with a washer underneath?

If so, using a blunt chisel or screwdriver, tap between the alloy sump and the sump plug a couple of times before trying to undo again. They tend to grip the same as oil filters do and I find that this sometimes helps get them undone.

If yours is the earlier one with the 13mm head and the small flange, once you have it undone, use a Mondeo/Escort TD sump plug washer (metal with rubber insert). This means you won't have the problem again and get a better seal into the bargain.