I have been contacted via a friend for help with identification of this coin. Pictures are not mine! I can't find a match in Spink - can anyone enlighten me? Apparently measures about 12mm across and weighs 2g. Owner assures me it is gold - the paler photo is a trick of the light.
appears to be a quarter stater of the carnutes: https://www.sixbid-coin-archive.com#/en/single/l31078170
Thank you for that - I have passed the information to the current owner. No wonder I couldn't find it in my British books!