VW Caddy/Seat Inca sdi full revs on its own?

Hi my Seat inca van started making an intermitent diesel knockish sound the other day and seem to loose power then it cut out and wouldnt start for a while now when i do start it up, it immediatley revs flat out on its own and i need to stall the engine to switch it off as the key wont do it.. it doesnt appear to be running on oil from the sump any other ideas or soloutions would be appreciated....