I have been trialling a new adapted laundry liquid for a few months now and can recommend it. The full recipe for it came from here.
Take 1/4 cup (50g) of washing soda crystals and dissolve in 1/2 litre recently boiled water:
Before use, gently shake to distribute contents and oil, then use 1/4 to 1/3 cup per wash. Current cost per 1/4 cup is 5.9p per wash or 1/3 cup 7.9p per wash. Pretty economical!