Turbo conundrum!

Hi guys, I've got a right niggly issue occurring!
My engine is perfect, except.... When the truck is first started in the morning as I'm driving the turbo spools up fine but dumps pressure at around 0.5 bar. It doesn't hold the pressure, it all goes. As the commute progresses the point at which this happens rises to around 0.9 bar. This is hugely annoying.
Has anyone else experienced this? I've checked all the inter cooler pipes, the inter cooler and the egr blank, all fine. And replaced the small bore hose from manifold to waste gate.
Any ideas?
Many thanks.

sounds like the wastegate sticking.

Thanks Tim, with fear of sounding like a mechanical novice, (which is technically correct!) is there any way of testing the wastegate?
Any ideas on where to source a new one?