Brief, digitally delivered relaxation task intervention, comprised of:

Two sessions guided (remotely) by a researcher. Sessions comprised of:

(i) relaxation exercise task using instruction components from progressive muscle relaxation

(ii) accompanying information, including rationale for the brief relaxation exercise task.

(iii) option to use the brief relaxation exercise task after the session (self-administered); option to engage in up to 6 guided booster sessions supported by a researcher.

A 9-item questionnaire assessing depression symptoms and their severity. Items are scored from 0 (i.e., not at all) to 3 (i.e., nearly every day).

A 7-item questionnaire designed as a screening tool for symptoms of GAD and their severity. Items are scored from 0 (i.e., not at all) to 3 (i.e., nearly every day).

6 self-rated items assessing the frequency of unwanted memories of the trauma in the previous week (on a 7-point scale, from never to many times a day) and the level of distress, nowness, reliving, disconnectedness and whether different triggers are associated with unwanted trauma memories (on an 11-point scale, from 0 to 100). Higher scores indicate more unwanted memories, higher levels of distress/nowness/reliving/disconnectedness and more different triggers.

Assessed with six self-rated items anchored to the past week: two items assessing concentration difficulties in general and due to intrusive memories (11-point scale; high scores indicating more difficulties), one item assessing duration of disruption after experiencing intrusive memory (<1 minute to > 60 minutes), two items assessing sleep disturbance due to intrusive memories (11-point scale; higher scores indicating more sleep disturbance) and one item assessing the degree to which intrusive memories affected stress levels (0 = not at all; 10 = very much).

Assessed with two self-rated items, one assessing the degree of distress associated with intrusive memories and the other assessing how vivid they were in the past week (both rated on an 11-point scale, 0 = not at all; 10 = very distressing; 0 = not at all; 10 = very vivid).

Assessed with four items. One question is self-rated: "Have the intrusive memories affected your ability to function in your daily life?" on an 11-point scale; higher score indicating greater impact on functioning. The other question is open-ended: "How have the intrusive memories affected your ability to function in your daily life in the past week?"). We will also explore whether there is a link between the content of participants' intrusive memories and specific domains of functioning - in open-ended discussion and using questions (asked at baseline) such as: "Are there any specific activities that you would like to do, but avoid because they are linked to your intrusive memories?" and "What change in your intrusive memories would make a meaningful difference in your life?" along with (asked at follow-ups, after the intervention/control task) "Since starting the study, are there any specific activities that you used to avoid (due to your intrusive memories), but now do more often?"

A self-report questionnaire assessing functional impairment across three domains: (1) Work/school, (2) social, and (3) family life. Domains are measured on an 11-point Likert scale, ranging from 0 (i.e., not at all) to 10 (i.e., extremely). The scale will be adapted to assess functional impairment associated with intrusive memories.

A 12-item questionnaire assessing difficulties due to health conditions, including mental or emotional problems (with reference to study event). Scores range from 1 ("none") to 5 ("extremely/cannot do"). Lower scores indicate better functioning.

A 4-item a patient-generated outcome measure of symptom-change after therapy/interventions, consisting of four items measuring three domains: problems (two items), function (one item), and well-being (one item). The patient chooses which symptoms or problems are most important to measure over the course of an intervention.

A single item assessing perceived health status on a 4-point scale (very good to very bad). The scale is reverse-scored; higher scores indicate better health.

A 7-item self-report questionnaire assessing the frequency of disruptive nocturnal behaviours (e.g., acting out dreams, episodes of terror or screaming) common in PTSD. Respondents report symptoms over the past month on a 4-point Likert scale, ranging from 0 (never in the past month) to 3 (three or more times each week). Total score ranges from 0 to 21. A clinical score ≥ 4 will be used, which discriminates between participants with and without PTSD.

2 self-rated items; one assesses the extent to which the respondent is experiencing poor sleep (with reference to study event) on a 5-point scale (from not at all to very much), one assesses the number of nights in the week the respondent experiences sleep problems on 5-point scale (from 0-1 to 5-7 nights). Both 5-point scales are reverse scored (0 - 4) then summed. Possible total scores range from 0-8; higher values indicate better sleep.

8-items drawn or modified from both the Temporal Orientation Scale and the Zimbardo Time Perspective Inventory. Respondents rate how true statements are for them on a 5-point scale, from not at all true (1) to very true (5) (e.g., "My plans about the future are pretty well laid out", "It is best to live day-to-day").

Self-rated questions assessing acceptability and feasibility of the intervention/control tasks (e.g., ratings of the extent to which the respondent would recommend the intervention, whether they consider it to be an acceptable way to reduce the frequency of intrusive memories). We will also include two self-rated and one open-ended question about the experience of using the digital platform to complete study procedures.

A self-rated item assessing self-reported accuracy of filling out the diary (0 = not at all; 10 = very accurately).

Participants will be asked whether they had experienced any adverse events with one open-ended question: 'Have you had any health problems since the last contact?'