Feasibility outcome | Details of feasibility outcome | Success criterion |

Peer mentor recruitment rate | Number of peer mentors trained divided by the number individuals who were eligible for the peer mentor role | 1. Demonstration that the intervention can be delivered in practice and is acceptable to participants and peer mentors* |

Peer mentor attrition rate | Number of peer mentors who were not matched with a participant divided by the number of peer mentors trained | |

Intervention completion rate | Number of participants who completed at least three peer mentorship sessions divided by the number of participants who commenced the intervention | |

Participant recruitment rate | Number of participants randomised divided by the number of individuals who were eligible to participate | 2. ≥60% of eligible individuals recruited |

Attrition rate overall and in each group | Number of participants who did not complete the 6-month questionnaire divided by the number of participants randomised | 3. <20% participant attrition with no evidence of attrition bias |

Sample size required for a definitive RCT | Sample size calculation using the feasibility trial PIH scale scores assuming: 5% significance level; minimum power of 0.8; and 1:1 allocation ratio. The PIH scale scores were used because the PIH scale was chosen as the provisional primary outcome measure. | 4. Calculation of a sample size that is achievable in a main trial |

Questionnaire completion rate | Number of questionnaires completed divided by the number of participants provided with the questionnaires | None specified for these feasibility outcomes |

Intervention fidelity | Content analysis of the mentors’ session summaries | |

Cost of delivering the mentor training event | Estimation based on the events delivered by the trial team members accounting for: staff salary, national insurance and pension contributions; resources; refreshments; and peer mentor travel expenses; but not venue hire, estates and indirect costs | |

Cost of delivering the intervention | Estimation based on the volunteer coordinator and peer mentor costs, excluding the training costs |

*No minimum/maximum rates were prespecified as acceptable for this success criterion.

PIH scale, revised 12-item Partners In Health scale; RCT, randomised controlled trial.