The NHS pledges to provide services at a time that's convenient for you. Outside normal surgery hours you can still phone your GP practice, but you'll usually be directed to an out-of-hours service. The out-of-hours period is from 6.30pm to 8.00am on weekdays and all day at weekends and on bank holidays.
Consider calling NHS 111 if you urgently need medical help or advice, but it's not a life-threatening situation. You can also call NHS 111 if you're not sure which NHS service you need.