Just graph the actual boost pressure it shouldn't be exceeding 2.4/2.5bar if it is then your possibilities are wastegate is stiff/sticking, the actuator hose is stuffed, the wastegate actuator diaphragm is stuffed or the map sensor itself is faulty.

You got it with the first suggestion.

Yesterday I checked the boost pressure with AlfaOBD, the BT adapter you suggested and a Hudl2.

The boost was exceeding 2.5bar at full load, exactly at the moment the EM light came on, and dropping below about 2.3bar as it went off.

I got the van up on ramps and got an adjustable spanner on the wastegate lever.
When I tried to move the lever it was stiff right near the start of its travel but, once I wiggled it a few times it freed up.

I've just done a 260ml run today, no EM light and the boost pressure never got above 2.2bar.

It looks like your patience has paid off, job done and thanks for the help.

I'm toying with the idea of buying the full version of AlfaOBD, but might have to sneak it past the missus when she's not looking.

Thanks again.

EDIT: Is there some way to mark this thread [SOLVED] ?