Yikes!And the sand wont be cheap! 200m at 200mm thick (100mm below cable and 100mm over the top) and say 200mm wide trench = 8cu m (8 builder bags worth)!
Just looked at that on some web site. Even at 5cm surround (10cm x 10cm) it is saying 5 bulk bags at ~£50 each. Going to a safer/easier to pour in 20xm x 20cm (i.e. 10cm safe margin) it is 4 times that and almost £1,000
Might be cheaper to look at 90mm or 110mm twinwall duct? 200m of that is around the £400 mark.
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