Maybe this is the building I'm thinking of.
Yes, thats the one. According to the article, it opened on Monday 17th June 1901 at 5am, replacing an earlier building, which was demolished, due to the widening of the road bridge. It remained open after the loco staff had moved to Old Oak Common, it closed about 1911/1912. It was converted into a Deeds Office by the GWR, and the upstairs rooms at the back were converted into a flat, for the caretaker and his wife, who remained there for over 30 years. British Railways finished with the building about 1966.
It goes onto say that it was due for demolition to make way for an electricity sub station for Crossrail, but due to local protests was saved.
An early photograph shows the lettering in the pediment:
THE GREAT WESTERN RAILWAY
COFFEE TAVERN COMPANY LIMITED.