Neil Williams wrote:

...which TfL get to keep and earn interest on.

Remind me, Neil - how much interest were you going to get on this fiver
that TfL are getting to keep?

Hint: Look down the back of your sofa. You'll probably find the mythical
lost interest there, and more besides.
--
http://gallery120232.fotopic.net/p12119124.html
(150 134 in blue with a bag to keep the snow out of the coupler, 2005)