The Ranco/SAAB bypass valve was not fitted until 2004, so a 2001 car is unlikely to have one unless the pipework has been replaced with the newer type.
As you have had the system drained for the water pump replacement (which to be honest is a pointless change on an engine that has only done 47k - the factory fit Airtex pumps are very robust, and rarely need replacement that soon), I would think you have probably got an airlock.
The K series coolant system can be tricky to refill avoiding air locks, and even dealers used to have problems (which is why Rover specified the use of a vacuum fill system). First thing is to make sure the jiggle valve at the cam belt end of the inlet manifold is clean and free (it is what allows trapped gas out of the head back to the expansion tank). You will see it when you remove the bleed hose attached to that end of the inlet manifold. It is possible to fill the system effectively by removing the bleed screw from the coolant rail and also removing the small rubber pipe from the coolant elbow (where the temperature sensors are). Also ensure that the cabin heater is set to open/full heat (otherwise the heater matrix won't fill). Then pour the coolant into the expansion tank steadily until coolant comes out near the coolant elbow. At that point, re-attach the rubber hose and continue filling steadily (tightening the bleed screw when coolant without bubbles is flowing out. Once filled, start the engins and keep squeezing the radiator hoses as the engine warms to help disturb any trapped air bubbles that may be left. This is the method many of us use, and I have not had air lock problems by doing so.
Another possibility is a failed expansion tank cap - it is not uncommon, and results in the system failing to pressurise, which in turn allows the coolant to boil at normal operating temperature.
