Try ringing your service provider, i.e O2, Orange etc. They can sometimes track it. A friend recently lot his, they managed to trace the signal but said it could be within a 10 mile radius of that point! Worth a try though!
NicolaB23, I lost mine when I took my friends dog for a walk in one of the parks near us and couldn't find it so make sure as Gard said telephone your mobile provider and if you have any money on it for any calls stop it straight away.