In both the Groupsave case, and the case of using a Network Railcard on GatEx services, there is clearly a conflict between condition 10, which states that the restriction must be shown on the ticket and the ticket restrictions, which apparately state that the ticket is not valid on Cross Country (or is restricted to certain operators only, which I suspect is actually the case). Until both are resolved, ie the restrictions are made clear at all points of sale and the operator restrictions are printed on the ticket, this will continue to be a messy issue.
Now, from what Ferret has written, I assume that Cross Country guards on the Southern have been withdrawing groupsave tickets and completing TIRs, which have then resulted in SWT being billed, presumably for not advising of the operator restrictions at point of sale. To counter this, SWT have put up posters advising of the restrictions which are in direct sight of the passenger at the ticket window, which I believe satisfies condition 1 (assuming that blind people are advised accordingley).
However, the tickets still do not have the operator restriction printed on them ('see restrictions' merely states that there is a restriction of some form, it is not the restriction), so surely Cross Country could carry out the same procedure against SWT as the tickets aren't printed in accordance with the NRCoC. Either that, or they are in fact valid on the Cross Country service to start with.
Now I have a question. If the groupsave tickets are not valid on Cross Country, that is presumably because Cross Country are not one of the companies promoting the groupsave offer. If so, Northern fall into the same category. Then please explain how you would complete the journey on a Barton on Humber-Lincoln groupsave ticket, stating the operator of each service you use and why your ticket is valid on each train.