Ok - got 2 quick queries about Pre-pay (which I now use and I've been
wondering about these things for a while now).

Firstly - if, after I've swiped in through the gates with pre-pay I change
my mind about the journey (i.e. forgotten something at home - have I left
the oven on etc.etc.), when I walk out of the gates again, would I get
charged?

And secondly, during those times when there's a problem on the tube and they
announce that it's recommend that passengers find alternative routes and
tickets will be accepted on other services, does that include pre-pay on
mainline trains that don't normally accept them (cos you can almost always
guarantee that the problem occurs when you're already on the platform so
you've already swiped in).

If the answer to the first query is that you wouldn't get charged, then the
second issue isn't a problem. But if you do get charged for a same zone -
same zone journey (£1?) then how would I claim that back?