OK, so I decided to be Brave, and do some highly risky testing earlier to see if my boost issue was mechanical or electrical.
Short of it is, it's electrical.
To test it, I was thinking about how the turbo worked, and how it boosted.
If the turbo was tired, it might not be making the boost it should. Equally, if the actuator spring had gone weak, it's possible that the pressure in the exhaust housing could force the flap open, so reducing boost.
How to test if this has happened?
REMOVE ALL BOOST CONTROLL - EEK!
Basically, disconnect the outlet from the N75 to the actuator. Running like this, the only thing that will limit boost is either the turbo reaching it's absolute peak, or the wastegate being forced open against the actuators spring pressure. No air is bled off to the actuator, and as such, it'll never open.
Now, seriously, DO NOT TRY THIS.
If indeed the turbo is tired, and it won't boost any higher, or the actuator sping has gone weak, then it's probably fairly safe, as the car will still be fuelling for the reduced boost level.
In my case, the test proved that the turbo was still good, with boost SHOOTING straight up to 25psi and holding there. my GOD it was fast :S
Clearly, the turbo is working, so my reduced boost must be something that's pulled back electronically.
The car shows no timing pull, so I can only assume it's pulling back boost due to a lack of fuelling. If that is the case, then running it at 25psi when it thinks it can only safely fuel for 16psi is a pretty daft idea! it has however given me something to hunt for, I know it's not mechanical, and I know the actuator is capable of keeping the WG shut up to 25psi (whether the turbo runs out of puff or the WG is forced open at this point I do not know, but it doesn't matter really either way).
It's worth noting that I only ran it like this for about 1 mile tops, I did 2 pulls in 2nd gear, and a single pull in 3rd gear. Each pull resulted in around 25psi of boost. It's seriously unsafe to run like this, so please do not try it!
So, electrical it is then!
Will start with a 4 bar LCR pump, then ignition and fuel pump relays, then perhaps look into a new MAF and N75 if that doesn't cure it.
Nice to know where to start though