Update from my travels today to come later tonight or tomorrow.
You can read selected issues by looking at my Twitter account: @bruceb90
Johnny, I tweeted Southern when I was delayed. They stated the ticket is treated as a single.
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My update, as promised:
Bletchley: Gate assistant didn't recognise ticket: "that's a funny looking ticket". No trouble though, let me through.
On train to CLJ: "That's great mate", as this conductor would normally say.
(No further checks whilst I'm travelling round Brighton/Newhaven)
Norwood Junction: LOROL staff didn't recognise it, but were very polite (perhaps the most polite I'd met all day, even calling me Sir) and let me through anyway.
Epsom: No issues whatsoever
Victoria: Two staff at manual gate. Looked like one RPI, one gateline assistant. RPI takes ticket and asks me to come to one side. She takes a good interest, asking if I printed it myself. She then noticed someone's Oyster had no credit left (perhaps a guaranteed PF compared to my more minor issue), so handed my ticket to the gateline assistant. He said "yeah, we get these occasionally" and let me on my way, much to the displease of the RPI.
Battersea Park: Once I'd found someone to open the gate, she looked blankly at it. On my re-entry, she asked lots of questions and said she assumed I should enter the number into a machine "or something". Let me in anyway.
Clapham Junction: No problems exiting (Brighton Yard). On entry (shopping centre), it was scrutinised and I was asked for ID. Credit card shown and let in.
Return train to Bletchley: As the conductor entered, the lady sat opposite said "your face looks like you're going to cause trouble"!! She was exactly right as the conductor was clueless to the ticket. She looked at it for a good while, before concluding with "I wish they'd tell us about these".
I effectively made three single journeys, each was delayed by 30+ minutes. Unfortunately, I had to cut out quite a bit of my plan otherwise I couldn't get back to Milton Keynes.
Would I bother again? For one or two trains, then yes. For any more, you're letting yourself in for a lot of hassle and potential embarrassment in front of other customers.