We have just discovered that the pottery drains taking rain water are all blocked. The blockages appear to be caused by builders rubble ( including an open rusty stanley knife)( roof work was carried out a few years ago) soil and plant roots. Attempt to clear the blockages with drain rods and spiral attachment have not worked, the drain rod alone can be passed from the well they should drain into up to three to four foot away from the down pipes, only six inches of the under ground drains have been cleared at the down pipe end. Can anyone advise? would hiring a pressure washer be of any help?