I agree, but I don't think that's readily available in the UK.Sounds like you could use a truckload of arborist wood chip mulch. Add it in nice and deep, and the chicken run will elevate above the water table; over time the organics and added presence of worms will further loosen the soil to help drainage - but mainly it's the elevation above the water you want. There are pics on this site of chicken runs using deep litter, surrounded by flood waters and the chickens are dry