The reason I asked the question is because I thought it would be treated as an £18 return ticket, so I expected to get £4.50.
I was surprised that it was only £4 and wondered how they'd worked that out. I'm not going to complain about the extra 50p, I just wondered if there was a different formula used for rovers.
Assuming that the ticket is used for one return journey per day of validity is fairly common practice AIUI. However even with season tickets different TOCs have slightly different calculations. For example, with a weekly season, some TOCs assume that you make 10 single journeys, whereas with some others, 10.5 single journeys.
There are some journeys where the NW rover is cheaper than the cheapest walk up single ticket. I wonder how someone would get on if they insisted they only used the rover for a single journey, and wanted 50% of the ticket price back after a 30 minute delay?
They should be declined imo. The option to purchase the single/return fare was available. The fact that they purchased an unlimited travel ticket is the only factor to be considered. The fact that it was only used for one journey is the passenger's choice, neither here nor there for the purpose of compensation calculation, for which purpose set formulae exist for consistency reasons.
Discretion, of course, is always available.