Charles
Klassmann is available from Quickcrop in 75 litre bags, but is very expensive for the domestic gardener: 250% more expensive per bag than when buying a pallet from JC Monro.
I have used it this season and results have been perfect, but it makes you focus on how many modules, what size of module you actually need.
I have used 3 bags through the whole season which cost a whopping £75 inc VAT and shipping.
But you can pretty much guarantee having no failures at all.