I just read the service manual regarding changing the coolant (section KH.3), and it says nothing about using a pressure tester. What instructions are you using that say to use a pressure tester?
Lotus Elise Service Manual's Instructions:
KH.3 - DRAIN/REFILL PROCEDURE
To drain the engine cooling system:
1. Remove the undertray from beneath the nose of the car.
2. Disconnect the radiator feed and return hoses from the front ends of the thro' chassis pipes, and collect the draining coolant. Remove the header tank cap to speed the operation.
3. Open the drain tap at the right hand rear of the cylinder block.
Note that draining of the heater matrix is not easily possible with the unit 'in situ', and that if draining for the purpose of coolant change, this volume should be disregarded.
To refill the system:
1 Refit the hoses to the feed and return pipes and close the cylinder block drain tap.
2. Remove the right hand front wheelarch liner and open the air bleed plug on the radiator outlet hose. From within the engine bay, open the air bleed plug in the heater return hose at the left hand rear of the engine bay.
3. Fill with the recommended coolant mix via the header tank and close the bleed plugs when a steady stream of coolant is expelled.
4. Start the engine and allow to idle, and periodically open the bleed plugs to allow any trapped air to be expunged. Top up the header tank when necessary, and fit the pressure cap when required to prevent overflow. When the cooling fans have cut in and then out, stop the engine and allow to cool. Recheck coolant level when fully cold.