Prices per litre, order what you need. 
Free collection at market or U.K. delivery.
A premium mix of 
Pine bark - used for orchid potting
Horticultural charcoal 
Worm casts
Perlite
Vermiculite
Coconut coir fibre
Ingredients in this mix will allow water to freely drain through, and allow transfer of gases at the root level. Switching your plants to this kind of potting substrate promotes excellent root health. 
Will not allow roots to sit in too much water, eliminates the risk of overwatering. Discard the water that drains through.