Ah. That would explain it. (Thanks to a 'Mr X' who PM'd me with an intriguing anomaly that probably doesn't need circulating). Will get to station early and do that. Thanks.
I know exactly what anomaly that would be

, I meant to PM it to you a while back but may have forgotten to send it - sorry! I would go with the anomaly as it is totally reasonable, cheaper, and much less hassle than faffing around with excesses.
If you asked for the Manchester ticket, booking office staff should offer this to you:-
Routeing Guide said:
Where two or more permitted routes are available for a specific journey*,
customers may wish to travel out by one route and return by another. If a
higher fare applies for the return leg of the journey the customer should be
issued with a ticket for the more direct route and an excess fare issued to
cover the difference in fare for the return routeing. This option should be
made available to customers who wish to pre-book a dual routed ticket prior
to travel.
* in this case, route: via Sheffield and route: via Leeds
That said, while many booking office staff do not seem able/willing to issue excess fares, unlike some other excess fares (eg. Std to 1st or overdistance) your right to a
change of route excess at half the difference between the fare paid and the appropriate fare
applies equally on board the train so if the ticket office won't issue the excess when you purchase it, then you can simply board anyway and you cannot be penalised.
The only problem with that, is at certain stations there are ticket gates in operation where the barrier staff may insist on the excess being purchased before allowing you through. For example although a passenger on a Rte: via High Wycombe ticket cannot be charged any more than half the difference to the
appropriate Any Permitted fare on Virgin, the gateline staff will likely insist on the excess being purchased before allowing you access to platforms, this delay can cause you to miss a train potentially.
So, you are entitled to an excess and that excess can be bought on board at the same price, but it's easier to buy the anomaly ticket that avoids the faffing about.