That is correct mate the 3 port seems to be the method of choice as far as aftermarket boost control is concerned, I run an Apexi AVC-R and cannot fault it. Neil, your boost problem is directly linked to your MAP sensor, if you have a dodgy sensor or it is dirty inside preventing it from sensing the, ecu goes into a safe boost mode (by safe mode I mean it stops the boost solenoid from operating and you run off wastegate pressure roughly 0.5BAR) and the check engine light comes on, you can simulate this fault by unplugging the MAP sensor, it is also possible that the pressure switching valve (the other sensor that is attached to the MAP sensor) has a blockage or one of its pipes is blocked?
If attempting to clean this system out use only carb or clutch and brake cleaner as it vapourises after use.
Good luck with sorting this mate.
Cheers Iain