Hi Caroline - just back from 2nd year in Pamps and booked again for next year. You need to layer up, as Jenny says. Pamps can be really warm when the sun's shining, but temps drop pretty sharply as soon as it clouds over. Thermal base layer, fleece mid layer and jacket/salopettes. You also need hat, gloves and socks (get some good ones or your feet will suffer!). Wher are you staying?
Yes Lizzi forgot the socks...and if your feet sweat like mine when i'm having a scary shit I cant stop moment you will need a few spare pairs too.
One last little tip - we took a small hip flask and filled it with Drambuie every morning - fantastic to warm you up when on top of the mountain, slip a bit in your coffee/hot choc or just for a bit of dutch courage when you need it!!