it would be impossible to dig a trench 2 foot deep, i wish! the clay starts about 6inches down and is mixed with shale in layers, all my crops are grown in raised beds that are made with 18inch cedar boards, and i have them filled with compost, manure and leafmould taken from the beech and oak woods near my place, i top them up regularly with more compost, its the only way. i think i will dedicate a permanent bed to artichokes tho, and do as you say, cheers or the advice.