Depending on how stained they are you could hose them down in backyard with a hose and broom or soak them in the bath for a couple hours first with a vinegar solution. They are buggars to clean. Good luck

Thanks so much everyone, I’m going to try some of your great tips this weekend!
Wish me luck!

My mum used to get the venetian blinds and put them over the clothes line and blast them with the hose and then scrub them with a dish brush and dishwashing liquid.
I detest venetian blinds so ive never tried this. Its possible that the blinds back then were stronger then the ones today.

sounds to me that there were smokers in the house, all I can suggest is put half bleach and water into a spray bottle [ take the blinds down and lie onto the ground ] then spray away, a bit of a scrub, then hose off,,,,, I hope this helps a bit

If you are anywhere near an ALDI store get their power force cleaner, (around $3.00 for a litre) A little on a damp cloth and grip both sides of the slat at once and slide along slat. Do about 6 at a time, then with a clean damp cloth go over the ones you have just wiped with the power clean. I used this on some i had and found they were light blue with silver flecks through them so prepare to be surprised! Power clean also cleans the moss stains off concrete. It is safe for plastics too.

I would take them all down in one go lay them out on the grass in the backyard and wash them with warm soapy water brush them down with a dust-pan brush on both sides and hose off. You could then hang them using old stockings tie them onto your clothesline to dry or final rinse. If they then need bleaching (maybe the cords do) then soak them in the bath with Napisan or Vanish or bleach.

I recently done my mum’s. l took them down and soaked them in the bath with some bleach .l was amazed whilst soaking the build up of dust just wiped off, they came up like brand new