No Service. Phone keeps losing signal for long periods.

My HTC One SV has been gradually worsening, it loses signal for periods of time that began as a few minutes but which now stretch over 24/48hours.

The phone signal tends to be weak when I have it but it quickly falls back in to a 'No Service' state.

Unclear what is the issue as I have no experience in repairing phones. Any advice much appreciated.