There is case law. In general, if you fork over the money then see unacceptable conditions attached you can get an immediate refund, and no harm is done.
Thompson v London, Midland and Scottish Railway Company
However, that predates a great deal of consumer protection legislation, so the situation now probably isn't 100% clear.
The interesting point about that case is that the passenger lost, because the existence of the conditions was mentioned on the rear of the ticket in a legible size, and the conditions themselves could be acquired at the station. Having the conditions available only on the internet is certainly a step away from that scenario (consider a passenger buying a walk-up ticket who lacks immediate internet access).
However, I don't think anyone's quoted what the NRCoC says about its' own availability:
[Your rights - a summary] This includes making these Conditions and the Byelaws available wherever you can buy a ticket and making it clear who to contact if things go wrong.
...
[Part 1, A, 1, d] Train Companies will ensure that you can be provided with or have access to the conditions applying to any ticket you buy, as well as the Byelaws, before you buy your ticket.
I think it's pretty clear that you are not immediately entitled to a printout to take away (although it's obviously good customer service)
provided you are simply given "access" to them instead. (Presumably acceptable methods would include a paper copy stapled to a public noticeboard, or being handed a copy to read at the counter and return?) However, if they have not allowed "access" then they absolutely must "provide", whether by leaflet or printout.
Where this might be of use to Martin is that in the context of your enquiry to the DfT, the Manual does not appear to go far enough to meet the requirements of the NRCoC - telling passengers to get it online, or have it posted, clearly does not
ensure you get access prior to being sold a ticket. The NRCoC, and the charters, are clearly attempting to meet the dictates of Thompson vs LMS, while The Manual is not.
Also, if a member of staff is proposing to sell you a ticket, the NRCoC requires them to provide or give access to the NRCoC prior to doing so, rather than you having to carry a copy that they will then rubbish. I'm assuming you are being told that your ticket is invalid and being asked to buy a new one? If so, they must be prepared to whip out the NRCoC or lead you to a copy prior to the transaction taking place.