Neil, you'll need 2 adaptors, one to mount your Canon lens onto the Pentax fit tubes and another to mount the Pentax fit tubes onto your Canon body! Pretty sure that you'll lose all electrical connections and IIRC, setting an aperture without them is awkward on Canons. Probably work out almost as cheap to part exchange your Pentax fit tubes against a set of EOS ones for your Canon. That should also restore the electrical connections and thus give you AF (although for macro, MF is often a better bet) and aperture control.