Hi ​@smithjp_uk 

Thank you for getting in touch with us. I am so sorry to hear that you are facing this issue.

Please use the following link to check the coverage in your area:
 https://www.idmobile.co.uk/help-and-advice/coverage
or try to reinsert your SIM, or try your SIM in another phone. This will determine whether the issue is with the handset or with the SIM card itself.

Or try to reinsert your SIM or try your SIM in another phone. This will determine whether the issue is with handset its SIM card itself.

If the issue persists, please try the following steps for iPhone:
iPhone (iOS) – Reset Network Settings
Go to Settings → General.
Tap Transfer or Reset iPhone.
Tap Reset.
Select Reset Network Settings.
Confirm the reset when prompted.
Once complete, return to the home screen and restart your iPhone.
 Samsung – Network Settings
From the Home screen, swipe up to access Apps.
Go to Settings → Connections → Mobile Networks.
Tap Network Operatthat youors.
Turn Select Automatically on or off.

Network settings for Samsung: 
Go to Settings
Tap General management
Select Reset
Choose Reset network settings
Confirm by tapping Reset settings

Thank you,

Zandile