There are three characters on Peep Show named Simon:
Simon is the husband of Sarah and brother-in-law of Mark Corrigan. We never see him, but he is referred to in the episode Sistering, when he and Sarah separate.
Simon is also the wealthy, attractive boyfriend of Dobby who is shown in the episode Man Jam.
The third character is Super Hans, whose real name is revealed to be Simon, in the ninth series episode Gregory's Beard.