I recall seeing somewhere a rule of thumb on how much drain bedding is needed, but can’t find it on the drainage pages.
I don’t wish to have to pay for skip space to take away more soil the needed due to ordering too much drain bedding, but given that the delivery charge is close to what half a ton costs, I don’t wish or under order.
Assuming you are using plasssy pipe 100 ml 10 ml gravel below and up to crown of pipe. You can use selected fill over the pipe as long as it is free from clay, vegetation etc: also depending on depth will determine if concrete surround needed. Go to building regs part h for more info.