2008 Peugeot 308 speedometer running slow

Hi there my name is yorkie,I would like some help with my vehicle.I bought the above vehicle two weeks ago from a dealership in Leeds.On the way home to Sheffield I decided to try the vehicle out on the motorway passing everyone on the motorway to my suprise looking down at the speedo I discovered I was only travelling at 70mph it seemed as though I was going faster than this,when I got home and switched off I looked at the speedo and saw that the needle wasn't on zero but below,so when the vehicle is in motion and when the needle is on zero the vehicle is travelling at 10mph so when the needle is on 70mph the vehicle is doing 80mph.I have taken the vehicle back to dealership and they have told me the speedo is working ok.I left the vehicle with them and told them I was not going to take the vehicle on the road with a defective speedo.I asked them to change it under warranty but they refused just saying the speedo is working fine at the moment it is stale mate and don't know what to do next.Have any you guys had this problem or give me some advise,I don't want to get in touch with solicitors just yet.Regards Yorkie