Mine was 21 cubic meters! So at that price £2100! I paid two builders with an electric cement mixer £800!That's not right somewhere, 21 cubic metres of c20 general purpose concrete would need 6720 kg's of cement, that's 269 25kg bags, at my rock bottom buy in bulk price that's £873.60p alone ! Either the calculations were out or all the gear was nicked !Are you sure you mean 21 cubic metres, or 21 square metres ? 21 cubic metres is a big slab, approx 14 metres by 10 metres at 150mm thick or 46 foot x 32 foot
For the record, 21 cubic metres of c20 would need
6720kg's cement
13250 kg's