You can't get a refund on a ticket that has been used (unless delays occur and you immediately return to point of origin because it's no longer practicable to make the journey).
I certainly don't agree with this.
For example, if the customer buys a single, and the train makes the majority of its journey, and then it has a long delay at a station en route due to a problem ahead on the line, the customer obviously has the option to abandon their journey at that station and continue by some other means (lift, bus, taxi, etc) of their own, even if the train company asks them to wait.
The railway industry isn't then at liberty to charge to include the part of the journey which they didn't provide. The customer does not have to sit and wait for an indefinite period of time for the train to continue through the disruption, or for the company to arrange alternative transport - although they may wish to in order to maximise on their entitlement to compensation. Asking them to wait when they don't have to and then declining to pay any refund in respect of the difference in services bought and services used is a clear breach of the contract.
Looking at the OP's specific case, it would be completely illogical for them to be ineligible for a refund. This is because they would have been clearly permitted to buy a replacement ticket to their new short destination, use that one instead, and then apply for a whole refund on the original unused ticket. Doing that would be crazy because it would create unnecessary administration costs on both sides, but it would certainly be permitted and it would certainly get the OP the difference in fares back, if that's what they wanted.
In any case, there will be plenty of situations on long journeys where, due to disruption, it is neither possible to continue to the destination nor to return to the origin. Nobody can seriously argue that a refund is unavailable, or that a fee should be paid to claim a partial refund in these circumstances. The conditions have to be interpreted to reflect that.
What I would do in this situation is write to the train company who caused the delay explaining the impact it had on my journey and asking them to decide what to do to make it right. If they decided that the answer was nothing, then I might pursue a refund claim or chargeback.