Hi Wildwood
Sorry I typed an essay and then my iPad ran out of charge and I lost it.
Lay the blind face down.
Untie the two outside cords from their bottom loops and get rid of them.
At one outside edge, nick each pocket and remove the rod. Cut 30cm off each rod. You can use garden secateurs for this.
Using a pencil and ruler, mark 12cm in from EACH SIDE every 15cm. Join these with the ruler and cut off 12cm from each side of the blind, going through both thicknesses of fabric.
TRIM 1.5 cm off the lining at each side. You will have to carefully unpick a few stitches at every fold of the blind. Unpick as few as possible.
Make another set of marks 1.5cm from each edge and press in a hem of 1.5cm the length of the blind.
Fold in again, pin and hand hem the blind on one side.
You will have to finagle the top and bottom hems so that they are unpicked slightly, re-folded and then sewed.
Insert the shortened rods into each pocket and repeat the hemming with the other side.
Your strings should still be in place.
What I haven't addressed is the too big fixing at the top of the blind.
I can deal with that in another post.