Is this the song which starts:

"British Railways are the best,
Their toilets flusher than the rest,
But please refrain from using one right now."?

I've just tried a search, but have a feeling they don't work for backdoor entrants. I think it might be in the digitrad under "Gentlemen will (perhaps should) please refrain from flushing toilets while the train is standing in the station" or something similar.

Of course I may be on entirely the wrong track here, it would be appropriate, any way if that doesn't help I will try to dig up the goods when I visit my reference section at the weekend.