Apparently, it has nothing to do with Stamp and Christie but people assumed that it did.

While the Waterloo station part was almost certainly embellishment on Davies part, it's almost certain it's of Stamp and Christie that he was writing about. Daview himself never discussed it until about a decade or so ago when he started denying it
Stamp and Christie were together from mid-late 66; the song was written sometime in the winter of 66/67 and released in Spring of 67
http://www.standard.co.uk/lifestyle/esmagazine/the-life-and-loves-of-terence-stamp-8588325.html

I can't say for certain but I don't think it's meant to be Stamp and Christie in the song.
I can't imagine two major film stars arranging to meet at Waterloo Station every Friday night.
And I think Stamp was going out with Jean Shrimpton at the time the song came out.

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Almost certainly yes, Stamp and Christie were a couple in the late 60s when Waterloo Sunset was written; and it was widely acknowledged that it was those two that Davies was referring to.
He has said in later interviews that he was NOT referring to Stamp and Christie and that those were two names he picked at random, but given this has only come up in later years, this seems like revisionism on Davies' part