Paul
CPA prices become ever more reasonable the more you buy. I have not bought but did research prices for horse manure and 2.7 cubic metres (£145) is not radically more expensive than 1 cubic metre (£115), as much of the cost is the implied delivery charge. Mushroom compost is slightly cheaper, £125 for 3.0 cubic metres. A mix of farmyard manure/mushroom compost is £140 for 2.7 cubic metres. For some regions of the country (not for me) there is an excess delivery charge which may be up to £20.
I have not enquired on prices for loose deliveries of lorry loads, but I suspect that is the relevant volume needed for a full 10 rod allotment.15 cubic metres for a first treatment of 10cm then 7.5 cubic metres for 5cm thereafter would cover 150sqm of growing beds.