To be fair, I think that National Rail Enquiries itself is just about clear enough. For example, if I search for a journey from Rochdale to Buxton, a large warning exclamation mark appears, over which I am told that my ticket may not be valid. I don't think that's quite adequate, but it is close to being there - really, that warning text should be automatically visible if the site is showing any journey which can't be completed on one ticket.
However, if I either click through to buy on Nothern Rail's website, or search straight through Northern's site, a no time am I told that the ticket is not valid for the whole itinerary being shown. I don't think that's reasonable and if I was travelling, particularly with a printed itinerary, I'd expect at the very least that Northern would refund any 'standard fares' incurred.