just fitted my new towel rad after draining the system, now going to refill it and will bleed rads in doing so, and manual air vent next to the HWSV. but how do i bleed my pump?! It is a grundfos 15-50 and has a circular slotted screw at the front, but no screwdriver (even a big one) fits it and allows it to turn! any tips please ... ?