I really don't agree that trying a different pen on the ticket, at a different time and in different conditions (he may have wiped it first to clear any grease, for example) help proves anything with regards to an accusation of tampering.
I'd be shocked if a magistrate took a test like that as any sort of 'proof' that any and every pen will work fine. I can run a cheap bic biro over a range of tickets and it will write fine on some and not on others, and if I go over and over the same bits - sometimes the ink will flow, sometimes it won't. Same pen!
I bet if the RPI had tried and it hadn't worked straight away, he would have just muttered 'not to worry, that's not a proper test anyway' and carried on.
As it happens, the wife plays tennis with a magistrate so next time I see her I'll ask what she thinks about this, in theory at least.
Let me finish by saying I am sure there are many people who tamper with their tickets and I saw a lady that was held until BTP arrived at Finsbury Park because she'd been observed over a longer period of time (and her ticket didn't operate the gate as it had clearly been used), but they would need to do far more detailed investigating than simply showing they can get a pen to work on the ticket! To have any faith in our legal system, I have to assume no court would side with the train company on such flimsy evidence.
You seem to be missing the point entirely.
It was
you who raised the idea of alleging that the RPI testing a pen on the ticket could be said to be 'tampering' with it
My point is that it would be considered no such thing.
ONLY alterations to the material details printed on the ticket will be considered 'tampering with its' validity'.
This means alterations to the day, date, time, place of journey, fare paid, discount indicator, class of accomodation etc.
All of that is irrelevant in a sense, because the rules of issue and use make very clear that if the ticket is not made out clearly, rail staff who genuinely believe that the ticket may have been altered, have the right to retain railway property for enquiry.
The fact that a Penalty Fare notice was not issued raises no real problems either. If a TIR or MG11 is submitted the TOC will send a verification letter to the traveller and that traveller then has every opportunity to appeal, giving their explanation by letter.