It was a bit of graffiti visible on the way in to Paddington. The wall was demolished in about 1980 apparently, the word 'where' was visible for years after. I never saw the original (apart from the 'where').
Then it appeared again years and years later, in the late 90s. I remember seeing it again the first time I went into London after my father died. I think it has gone again, but obviously I was struck that it had been remembered and recreated over 15 years after its first manifestation.
Here is one of the better references of the 125: here.
The phrase has obviously seeped into public consciousness in a small way. It is mentioned in the book 'Notes From Overground' by Tiresias, the random jottings of someone commuting by train into London from Oxford. I have recently repurchased this book via abebooks as my original copy has disappeared or become lost deep in the Vortext. On the subject of this grafitto, this book says 'curious discovery that the first six words of [...] form the first line of a poem by Robert Graves entitled Song of Contrariety. Nowhere in the rather opaque poem do the last four words appear. The information only compounds the mystery'.
The book was published in 1984. It can be viewed as another psychogeographical tome, rather ahead of its time. Such books do serve both as a terrible enticement and a terrible terrible warning, in that the form is so alluring but yet obviously so hard to pull off.
Amusingly, it contains a number of things 'overheard' on the author's train journeys. One of them is
1st Student: 'He's probably not as old as he looks.'
2nd Student: (with great seriousness) 'oh, yes he is, he must be at least 23.'
I am sure that the 'discussing x's older boyfriend' conversation, which I guess this could well have been, is hardly rare but I like to think that this was me and Chris Hamblin (with me in the role of 2nd Student), discussing Huw Mascetti :-). Probably given the gestation period of books we would have had our conversation too late to be thus recorded, but I do remember us going up to London by train and having that conversation. (now there are names you don't hear often!).