[5][6][7] He has written for film, television, theatre and radio, including the screenplay for the ITV six-episode comedy-drama television miniseries Married Single Other[8] and the 2024 Netflix and Locksmith Animation film That Christmas.

Souter began his career in advertising as a copywriter at Abbott Mead Vickers BBDO in 1991, where he worked alongside Paul Brazier for 4 years.

[11] Souter has written plays for radio, theatre and television.

His radio plays for BBC Radio 4 include Puddle,[12] Goldfish Girl,[13] Stream River Sea,[14] That's Mine, This is Yours,[15] What Love Sounds like,[16] and "13a, 13b".

[17] Souter's television drama, Married Single Other, is a series based on the lives of group of people who are either married, single or "other", other being defined as in a relationship.