Have you tried a different charger and lead?
Have you carefully cleaned the charging socket with a small pin?
If yes than if the phone is under guarantee you can either return it to O2 or use the manufacturer's own repair service.
https://www.o2.co.uk/help/device-and-sim-support/device-repairs
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
