The Flying Banana tour is the name being given to the railtour which is being run by GWR on Saturday 1st of June to mark the end of the HSTs on the GWML. Yes, it's a while after the last ones leave Paddington in regular service, but it will be quite the event as it will run around all of the lines which are served by HSTs. It is called the Flying Banana because when the HSTs were introduced the yellow ends were a lot more obvious and apparently they had that nickname, which is now used for the Network Rail New Measurement Train.
-Peter
EDIT: Tickets went on sale at the Long Rock Depot Open Day and I think that they have sold out. They were really expensive, even though it was a long journey, and I'm sure people will be selling them online for a lot more than the original price!