Here in the states, we always have to have the backing of the FAA. If you crash and can point to something in an official government document, they will be less likely to blame pilot error. So here is the FAA take on it, The glide distance with no flaps will be farther, but the loss of lift, and the increase in stall speed related to retracting the flaps is not worth it. If you loose the engine with no flaps, keep them up until the landing is assured to maximise your glide distance. The Fed's have a little picture on glide distance with and without flaps at the bottom of
page 8-3. They also say "
Never retract the flaps to correct for undershooting since that will suddenly decrease the lift and cause the airplane to sink even more rapidly."