Drills can often be obtained from your local tip for a pound each. The Nicad battery packs go and peoople throw them away, even though the drill itself is still good. This has the advantage that it is a lot easier to cut up a drill you got out of skip for 50 p or a pound than a brand new one from Argos. Get a matched pair. Post here if you need help in getting the chuck off - you need to remove the chuck to expose a 3/8 threaded shaft - the blue wheels just screw on to that shaft.