An update...
The engine has a lot of boost creep due to the position and size of the waste gate on the manifold, so I have ordered a much larger waste gate, and will ensure that the inlet port is angled well on the manifold to maximise flow. Once that has been done, it will be able to experience some proper power runs.
We saw 165kW at 4400rpm at 25psi with no fine tuning at all, so it is looking like it will be a healthy number at the end.


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38mm. I think the primary cause of the issue is exhaust manifold design. I have a 17psi spring in the waste gate.

This is why it boost creeps - bad manifold design. 
The gas has to turn more than 90 degrees to exit out of the waste gate. 
If the angle was a lot less, it would probably be OK.
 

Space is an issue, though I think we can use a 50mm with a better designed entry port from the manifold collector.

Haltech fixed the ecu yesterday under warranty. Apparently it wasn't assembled properly and there was stress on the solder joints of the map sensor, which eventually cracked the joints. It's now running again and it is ready to take to the exhaust shop on Saturday to get the manifold modified to suit a larger waste gate. After that, it will go back onto the dyno for the final tune. I hope that will be the end of the niggling issues. I'm started to get excited now. It's going to be so much better than before!

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I dropped it off in the morning and picked it up just after midday. Their service was pretty good.

Did the exhaust shop design and fabricate the manifold? A larger wastegate may not solve the issue if you cant get the exhaust gas to reach the wastegate in sufficient volume. Therefore I would suggest the designer to have a good think about how the exhaust gases pulse from each cylinder and how the gas flows through the manifold runners and merge at the collector.