Well lofty29, it dosent look as though you are going to get a responce here. From what I can tell from what you have said I would say the answer is no. Does the mains charger also plug into the USB socket or does it have a different one to use?
If it is a different socket, then you may be able to get a car charger that also uses that socket ,thus leaving the USB free. Then you could perhaps run both at the same time.