You’ll need to buy some Fuller’s earth which is a wool relaxant, available at the chemist.
For dark-coloured jumpers, fill a nappy-sized bucket with blood heat water and add 2 tablespoons of Fuller’s earth. For light-coloured jumpers, add 4 tablespoons.
Put the jumper in and gently agitate it with your hands until it’s thoroughly wetted. Let it sit for 10-15 minutes and then completely rinse in blood heat water. Don’t leave it for longer than this or it will bleach.
Put the jumper on a towel in a shady spot and dry it flat. Gently stretch it back into shape as it’s drying. To make the stretch more even, use 2 wide-toothed combs on either side of the jumper and stretch with the combs as it’s drying.
It’s not as effective but you could also use 2 tablespoons of epsom salts, instead of Fuller’s earth, to a bucket of blood heat water.