From what I can see, I think the best thing to do will be to remove and oil/grease the mechanism (as per DIY section on removing door straps). I guess this isn't so bad, at least it doesn't cost. Just takes a little time, is all. I'll have a go at it as soon as I can.
Badinfo, you've at least tried oiling the hinges and as much of the door strap mech. as you can?
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black