Skoda octavia bkd 2.0tdi

Does a fuel leak between pump and head mean internal issue and can this cause underboost fault code??

No, the fuel passes into the head from the pump so it is possible that the seals need renewing. I doubt it will be the cause of your under boost fault code

Kool tnx

Ive changed the gasket which has stopped the leak but am getting underboost.

Changed most things associated turbo, maf map, egr cleaned, vacuum check etc n still nothing