Graham J wrote to uk.transport.london on Thu, 6 Jan 2005:

I tend to top-up my pre-pay at credit card only touch screen machines at
tube stations when I occasionally pass through them. They tend not to be as
busy as other ticket machines.

The only snag is, though, that you can't (as far as I know) top up your
pre-pay AND add a pass in the same transaction, and then they won't let
you use the same credit card twice in five minutes.... You would have
thought they would ask you if you wanted another transaction before
asking you to insert your card, but, unless I'm being totally dim (quite
probable!), I don't think they do.
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 2 January 2005