There is a little plastic pin in the centre of the clip, you need to tap it in and through. The clip can then be pulled out. If you can find the pin you can re-use!!
thanks les! i had worked out that it must be the pin needing to be tapped through - but failed to find the right tool to persuade them to budge. *goes off to try again*
good to hear that they are theoretically re-usable!
managed to knock the pins through using a hammer and a small bolt as a punch. will have to try and remember to look for them once I've taken the headlining out.