Hi Peter, the timeout was set to 2400 in the web.config however the script is timing out well before that (usually within 30 seconds). We're not using any custom code for the import, just the in-built Kentico import tool.
Here's some more detail on the error:
Exception occurred: [DataConnection.HandleError]: Query: SELECT * FROM Newsletter_Subscriber WHERE [SubscriberID] = @SubscriberID Caused exception: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.