The roof on a VP type of wagon simply clips on using those very fiddly clips that you get on things these days. If your PH is the same and I think it would be you need to open all the doors and check with a torch inside it, you should find 4 clips from memory along each side but be careful as above the door ways all those types are pretty flimsy once the roof is removed. I found that starting at one end and working your way down the car worked well. You simply need to push out the sides on one end enough to disengage these clips and they are tight as well. Once you have done that then slowly work your way along the car doing almost the same thing slightly pushing each side out enough to enable the clips to disengage. You might find it better to do one side at a time and place some thin card or something into the gap where the clips where on that side and the disengage the clips on the other side, either way should work. Starting the first clip though is a real problem but a small screwdriver used inside the car to gently lever the car side away slightly from the clip could be needed to start it.
Just remember though that after removing the roof to clip it back on the right way though.