Power fade

I have a 2006 Octavia estate diesel automatic - it has 56k on the clock - goes great, good as new - apart from when it hits a long hill, or, for example, when reaching an uphill climbing lane when it goes so far and then power fades. It doesn't matter what I do with the accelerator pedal, nothing changes, but if kick-down then it drops gears to 4th, loads of revs but no more speed, still fading away.

Does anyone know if this is a faulty dump/pressure valve to do with turbo boost?