Garage inspected P/S pump and steering rack and found no obvious problems with either, not externally anyway.
I don't know if it's possible that something has gone wrong with the steering rack internally which can't be seen externally?
They have recommended I take it to a specialist wheel alignment place. They reckon if there is an alignment issue it could cause the weird power steering. I'm not so sure, but willing to give it a go.
The power steering still works, it's just very sluggish. I think we would all agree that when you crank the wheel when turning a corner, and suddenly let go, the wheel normally springs back in an attempt to centre itself. It does on every other power steering I've ever used anyway. But in my car at the moment, it doesn't do that. It may move back a bit, but it doesn't really get anywhere close to coming back to centre, you would effectively keep going the same way you cranked the wheel. It's as if the steering is in treacle! You would have to manually move the wheel back to centre position.
That's definitely not normal. I don't know what everyone else's Z is like but in the 10 years I've had mine, I've never felt anything wrong with the steering until now so I'm sure it's not just me being picky.