Hi Claus,
If you do try the hot water method make sure you don't use boiling water straight from the kettle, I tried it on a severely warbed Airfix 727-100 fuselage, the water was too hot and whatever stresses were in the plastic causing the warp just took over and I ended up with an even more distorted fuselage.
If you can clamp the parts in the position you want it might be worth leaving it in strong sunlight on a window sill for a little while, I tried this once on some vinyl tank tracks, not sure if it will work on styrene.
Happy modelling Ant