white vinegar, sprayed on, let it sit for a few minutes then wipe off.

Two words: power washer

If you don't have one, maybe you can borrow one? (They can be expensive to rent)

dettol mould and mildew remover its brilliant ! spray on and leave.

Have you used bleach? I have had success with bleach. But then now sell a mold stain remover at the home improvement stores that works well. I have also had luck with bleach and a magic eraser sponge. I would spray it with bleach water then go back with the magic eraser. If it is fabric chairs. I would go find a mold remover from the store. Here is the mold remover I have had the greatest success with. I live in FL where mold is a constant battle. But try the magic eraser if they are solid chairs. If there fabric here is a link with some ideas Good luck.

http://www.ehow…

DO NOT USE BLEACH! Use Ammonia, you know simple glasss cleaner. Or wipe on baking soda let it sit for an hour wipe it off, with water and mild soap.

Bleach does work. You need to wear old clothes as you will get spotty.
Wipe a strong solution on and let it work. Rinse off well and bingo!