Channel 657 (BT Sport 1 / TNT Sport 1) has been set on a one hour delay for the last few weeks and I am unsure whether it is my box or a general fault. Anyone else had this issue or know how to resolve it?

first of all, CH 657 is specific to your service.  Here we do not discuss specific services.

Keeping it generic, If your EPG is off ONLY on this channel, then the issue is on your providers end.   If the EPG is off by 1hr on ALL channels, then you can change the offset in the EPG offset section under menu->settings->configuration->EPG Time Offset.   

Normally Buzz devices will use the EPG time zone settings provided by the server, and local time zone to calculate the offset automatically (I never had to change it manually on buzztv 5).  But you can override it as mentioned above.  But this affects "ALL" channels.  Single channel issues, you will need to contact your provider.