Marcus I think Charles has some friendly farmers who sell him a lorry load of manure for a decent price. You can certainly source 5 cubic metres online commercially for £300 if you can’t find a better local source.
I am certainly thinking of 2 cubic metres of cow manure with 2 cubic metres of suitable wood chips as an overlay. Again that is around £300 online. That would hopefully serve my garden for 5 years…..