Good to see Haltech could sort it out so quickly. Did you take it to the head office or post it? Rob is working there so you could have said hello. 
Hopefully it all goes to plan with boost control. Certainly looking forward to seeing the results. 
Have you logged laps on 'race chrono' or the likes? It would be good to do an overlay with the engine spec you have run for years and the new one. Interested in seeing the differences of power delivery off the corner. 
Good luck!

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

B24
Rob is working there so you could have said hello. 

Not yet. I start on the 27th :-)

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.

Have you got a better pic of the manifold showing where the wastegate is connected? Almost looks like it is hanging of a single runner just before they merge?
 
Most turbo kits utilise an adapter that has the wastegate at 90 degree angle which work fine. My GT35R had a 38mm gate which held boost fine. 

It's a Doc Race manifold. The waste gate inlet is as close to the turbo as possible - i.e. as far into the open collector as possible.
The ideal design for this manifold would be 2 x 38mm waste gates on either side of the collector, and at an angle that promotes maximum flow into the waste gates.
I am going to fit a single 50mm waste gate on one side with a very good flow angle into the waste gate inlet. That should do the trick without totally remaking the manifold.
It's mainly the bad angle with the existing design that is causing the problem.

I've had no trouble controlling boost anywhere between about 17 and 30 psi with a 44mm wastegate and the exhaust manifold pictured in my avatar.

I'm hanging to see if the bigger wastegate and angle change fixes the problem too because when you look at the pictures on page 42 of this thread it really looks like only one collector is connected to the wastegate and definitely like the flow from at least two of the cylinders can bypass the wastegate. Will be interesting to see what happens after the weekend.

Here are some better photos of the manifold.
https://photos.app.goo.gl/gL7gVkChfp7JnUDn6

I think that looks ok as its catching two ports plus it will scavenge from the collector. It is also a proven manifold so I go back to saying as I did before, it may be the spring pressure but a 50mm will not do it any harm. If it becomes too much, sell it and fit a 2GR..