Well you just proved that they do.

I bet your name is on their dodgy persons list now.

The obvious thing is to franc with a sort of ink that glows in UV this will
then not obscure the wonderful pictures on stamps.
It also means that its harder for the public to know if its been sitting
around in a post office sack for weeks. I'm not a fan of Royal Mail at the
moment, They are pretty poor at getting things reliably to where they are
going in a timely manner.
Brian

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This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

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"john west" wrote in message
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I had a stamp stuck on an envelope that was not Franked and so thought had
never been used. So i cut around it and stuck it to a new envelope and
posted it. but i got a note from the post office that i needed to go in and
pay extra.
As an experiment i used a stamp that i knew for sure had never been used
but had been stuck on an envelope. So i again cut around it and posted it
to myself and it arrived.

Does the post office have some way of detecting if a stamp had been used
through royal mail even though there is no visible sign of Frank Marks?