Not sure which episode but I recall Foggy almost shouting Simmonite at him but that's all? Seymour seemed to refer to him as " boy with the wooly hat " once or twice.
I'm not sure if it counts in the same way, but I've been watching Seymour episodes lately and Seymour does refer to "William Simmonite, OBE" at one point in "Wind Power." And I'm pretty sure he uses "Simmonite" to call him on occasion as well.
I'm sure there must have been at least one time that Foggy used the name Compo, I just don't recall when.
However, when Foggy was first introduced to the show, in the scene where they met him at the bus stop, as Foggy is exiting the bus he looks at them and says; Clegg? (Clegg nods), Simmonite? (Compo nods).
Foggy mentions the name Compo in the episode where they drag Smiler in because he is jealous that Smiler is lodging with Nora
That is the only time I can recall Foggy actually saying the name Compo.
I think Foggy says Simmonite right when he gets off the bus in the very first episode he's featured in. I'm hard pressed to remember him ever saying Compo though.
At the beginning of The Great Boarding House Bathroom Caper, Foggy thinks he's forgotten something, realises what, and very quietly says 'Compo' to himself.