I always suspected my car wasn't boosting quite right the odd time. Finally figured out why - the vacuum pipe to the turbo was kinked. It runs from the right hand side of the engine where the vacuum pump is, goes under the plastic engine cover, takes a sharp bend then heads to the valve and then to the actuator. It was kinked at the bend on the corner of the cam cover.
Unkinked the bend and rerouted the pipe, and now it's definately boosting better.
If anyone is keen, I'll take pics tomorrow of what I did.