I have worked this one out and if any one else would like to know, please see below:

Setup an incoming mail record and add the relevant create new record in:

For this it would be helpdesk.

Setup a new server action:

I have named this re-open ticket, but anything that makes sense would be fine.

Within the Server Action, select the object you want to adjust. For my example, i am using Helpdesk. Action type is "Write Object". Under the "Create/Write/Copy", select status value is equal to open.

What this does is to set the value to open, on any ticket when a reply is received. Ok, the ticket may be in a different status and may in some circamstances not need to be changed. But at the moment, we need the ticket to move to open for any replied to tickets. Additionally, I will add a new status later, called perhaps "customer replied". So we can track when a customer has replied by status.