I don't think there is an automatic entitlement to an excess however you should approach staff at the earliest opportunity either on the train or at the excess window on arrival at Southport. Once you leave the station at Southport, the ticket has been used and would no longer be excessable (may also be swallowed by the barriers too).
Personally, I'd just pay the extra 10p at the outset.
I don't know why you don't think Star can, as I use Star at my ticket office and quite often excess singles to returns.
It's a combination of hardware and software - a ticket issuing system - but it wouldn't know whether the journey has been completed, that's where the human comes in.
Star is indeed a widely used ticket issuing system. The rules say that staff shouldn't excess tickets once they have expired (i.e. once a single has been used to its destination), but of course you may encounter staff willing to use their discretion to still issue you with the excess. No guarantee of that, of course. Star itself is just a computer system and doesn't know whether or not the journey has been completed; it will do whatever the person using it tells it to do.
I did it lots of times for people who got off a DOO train and came straight to the ticket office having bought from a TVM at the origin, and refused it lots of times for people who came to the ticket office several hours (or days, in some cases!) later. That was pretty much standard practice where I worked.
Yes I have, on the grounds that they bought the single at a TVM from an unmanned Station and hadn't had an earlier opportunity to change their ticket from a single to a return.
