Yes - the doc you download from Northern website surely provides the official answer - you might think....
Northern | Buy Cheap Train Tickets and Check Train Times
www.northernrailway.co.uk
In the FAQs for the offer it says as below - the link takes you to stations served by Northern (which includes ones run by other TOCs) - I tried it and Ellesmere Port comes up - so you should be able to buy it there:
The redemption form generated by the Northern Website says as below - so any staffed station where a Northern train calls - my bold:
This isn't vague.
and
The adverts in the published newspapers direct you to these web pages where it says "For redemption instructions visit..."
I suspect it's Merseyrail pretending they are not part of the National Rail network once again...and this issue should be taken up with both them and Northern Rail
I did also wonder if it's in the ticket database if in reality any staffed ticket office anywhere could issue them in theory - but they may be instructed not to. Any ticket office staff on the forum could probably tell us - if they work at a ticket office not served by Northern