My question is around how to structure a user story for a user filling out an online application for a personal loan. For complexity, lets assume that the user must enter 3 different sets of contact details (name, address, contact number, email, & role). They must also provide supporting documentation (pdf, excel, word documents) and they need to answer 20 mandatory questions.

My thinking is that I should firstly create a Epic with the following description: "As a applicant, I want to fill up the Personal Loan form, so I can apply for a loan". I then need to create separate user stories for the 3 different sets of contact details. I should then create 20 additional user stories for each of the mandatory questions (as each question may have different acceptance criteria) and a further user story to cover the uploading of the supporting documentation. 

You could create a feature for each section of the form (including that each feature must be on the same form) within the epic and then stories within each feature. That way if it is too big to create in one cycle, you can set a sprint goal as one or two features instead of the whole form.

Ken Rubin has a great explanation in his book 'Essential Scrum: A Practical Guide...'. 

Also, if you're using Scrum techniques.

The developers should pick up these details through conversations during planning, estimating or any other form of conversation you have. And then you can capture notes to each Story, Feature or Epic.