yes it's interesting because I honestly thought that the 'half the difference' concept was automatically the normal one. clearly i won't rely on that in the future then.
I can do an Off Peak GC Sgl - £35 - one way and an Anytime GC Sgl - £58 - the other way.
Would that be better than excessing the £61 Off Peak GC Rtn to the £95 Anytime GC Rtn?
By £2, yes.
£58+£35 = £93!
The best thing to do with excesses, is if you don't get offered the excess you want, shop around until you do (if you have time and if the ticket is of high enough value). If they try to charge the full difference ask them if it should be half the difference, if they say no, try elsewhere. Different systems are set up to do different things, some are wrong and some are right.
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Basically you want a to go off peak and return at peak time for £78, and you can achieve this by using the Mobitix site.
https://www.mobitix.thetrainline.com/GC/advancePurchase.aspx
Mobitix was originally designed as only valid on specific trains but GC appear to allow you to use it on other trains in the same day, the conditions state:
Section 2: Terms and Conditions specific to train companies.
Grand Central: Single journey must be on the date on the ticket and completed by the following day. Outward and Return portions of a return ticket can be made within one calendar month. Reservations are not essential but are recommended for longer journeys. The outward portion of a Standard Open Return ticket is valid only in conjunction with the unused return portion.
In the event of a conflict between the train company and the general terms and conditions, the train company terms and conditions apply.
This appears to indicate that GC are more lenient so you can use a different train to the one specified. However what is confusing is where it talks about "return portions"; I believe the tickets are issued as singles not returns, so you can't actually return within a month as the terms appear to suggest.