Is this decorative bark on a flower bed or a bark path, Bonzo. If it is on the flower bed then the old bark will be decomposing and benefiting the soil, so put new bark on top of the old. The old rotting bark will start the new bark rotting sooner than it would if the old bark were not there, so you might want to clear a footpath down to a clean, hard surface before putting the new stuff down. Having said that, in practice I just add the new bark or chippings on top of the old.

yes I add new bark on top of old bark.