Participants in the control group will receive information that is currently used routinely in the NHS site to inform patients with ACS what to do if they experience symptoms after discharge. The information from two leaflets: 1. 'Using GTN', produced by the hospital and 'Angina' produced by the British Heart Foundation, published 08/04/2014 and available at https://www.bhf.org.uk/publications/heart-conditions/angina . The information explains the symptoms of angina and heart attack and advises what to do in the event of experiencing these symptoms. This information will be presented in written text format on screen.

Participants in the visual intervention group will receive the control condition specified above PLUS a specifically developed Text+Visual BCT-based intervention, comprising the 12 BCTs identified earlier in a Systematic Review and expert consensus study. The BCTs are Problem solving; Action planning; Social support (practical); Social support (emotional); Instruction on how to perform the behaviour; Information about health consequences; Salience of health consequences; Prompts/cues; Credible source; Pro's & Con's; Comparative imagining of future outcomes; Mental rehearsal of successful performance

Information leaflet (usual care) plus text-only BCT-based intervention (Intervention group 2) Participants in the text-only BCT-based intervention group will receive the control condition specified above plus a text-only BCT-based intervention. This was developed in the same way as the text+visual BCT-based intervention but does not include the visual elements (i.e. animation). Instead, the voiceover from the animated film is displayed in text on screen instead.

Participants in the visual intervention group will receive usual care specified below plus a specifically developed Text + Visual BCT-based intervention, comprising 12 BCTs identified from SR and expert consensus study. An animated video, just under 8 minutes in length is hosted online in the Intervention Modelling Experiment. The animation contains 9 of the 12 BCTs and tells the 'delay stories' of three different characters. It was not possible to deliver all of the 12 BCTs comprehensively in the relatively passive media of the animation as some techniques require active participation from participants (e.g. action planning). Thus, n=7 BCTs (1 2 Problem-solving; 1 4 Action planning; 5 2 Salience of consequences; 7 1 Prompts/cues; 9 3 Comparative imagining of future outcomes; 15 2 Mental rehearsal of successful performance) are also delivered via short web-based exercises which follow the animation.

Intention: Informed by the Theory of Planned Behaviour (Ajzen 1991), participants' intentions to phone an ambulance immediately will be assessed in response to each scenario using a single Likert-type item ('For these symptoms, after this amount of time, I would phone an ambulance immediately') scored 1=strongly disagree to 7=strongly agree.

Participants' illness representations in relation to symptoms presented in each scenario will be assessed using the Brief Illness Perception Questionnaire (B-IPQ) (Broadbent et al. 2006). The questionnaire consists of 9 items assessing the five components that make up a person's perception of their illness - identity (beliefs about the illness label and symptoms), cause (beliefs about factors responsible for causing illness), timeline (beliefs and expectations about the course of illness), consequences (beliefs and expectations about the impact of illness) and cure-control (beliefs about the efficacy of treatment or coping behaviours). The questionnaire has good test-retest reliability, has been validated among people with Myocardial Infarction (MI) and has been shown able to distinguish between different illnesses (Broadbent et al. 2006).