In you r pages on LAnd Drainage for Fields and Gardens, you have some great diagrams showing what goes into the trench, but I can't find a description of the bottom layer - "Pipe Bedding". What kind of material is this ?
Pipe Bedding for Filter Drain
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It's usually a 20-5mm clean gravel, but, as different gravels are sold throughout the UK/RoI, the actual spec varies from region to region. Use whatever is reasonably priced in your area - as long as it's in the 20-5mm grading envelope (eg, a 10mm or a 15-8mm would be OK), and it's clean (no fines) it will do.