I'm relatively new to cossie ownership, When I first got my cossie (stage 3) it would peak at just over 1.5 bar and hold around 1.3ish - Now it barely peaks over 1 Bar.
Is there anything that could cause this to happen?

 

The weather, or a duff/weak actuator, wear in the turbo.
tabetha

 

As above.

 

Could be the actuator...
Admittedly I have tried to turn the boost up by winding the actuator back although this doesnt seem to make a difference whatever way I turn it?

 

It won't if it is knackered, could be the seat on the wastegate getting worn and leaking as well, VERY common.
tabetha

 

Leak in vacuum hoses possible

 

Be careful tinkering with boost, more than 1.5bar on a std fuel pump, original pump wiring and standard intercooler is probably asking for trouble.

I run an uprated fuel pump, wiring, 4x4 intercooler and T34 on our 2wd cossie. We've left boost at 15psi held or thereabouts to be safe because detonation occurs on a warm day, hot engine if we increase it much beyond that !

Unless you know what you're doing I'd make sure you have this boost issue checked by a pro, or, we could soon be trying to help you out through your first YB engine rebuild