I have been online ... and swapped the tickets for ones for tomorrow - it warned there might be an admin fee but I appear not to have incurred one. Will report back further. Probably easier as I had not collected the tickets but it has only let me go "mobile" - a new step for me though got used to it with Interrail so I suppose I can manage GWR▸ .
Delighted to report that the ticketing worked excellently.
I was told my
QR▸ code was only valid with the replaced tickets, so I collected them when I got to Newhaven Town (and the day after they were dated) but in practise only my QR code was needed. Also stated that I would need my old persons railcard but again that was not needed.
I am probably convinced to use mobile ticketing after trying it out in 20 counties in the last 2 months ... which, however, is not really good news for those who don't have phones or the confidence / knowledge to work out the public transport systems.