I have recently bought a 2010 Octavia VRS which has been well looked after completely original. I went yesterday to see about a stage 1 map for it and the owner said that it lacked the usual power for this car. He went on to say that he thought it could be the diverter pump. It is fine around 3000rpm but lower than that and it is almost pedestrian! Has anyone else suffered this problem and how much does it cost to fix?

You'll need to find someone who can do some diagnostics. A fault log read would be a place to start.  You could log boost pressure under WOT in say third and see if it is lacking below 3000rpm. If it was then it could be the diverter valve or turbo wastegate or even something else like loose intake pipework.  No one is going to be able to tell you with any confidence what it is with that level of info. You will just be getting guesses..

Thank you very much I will get somebody to do a diagnostic check and a fault log read. Many thanks