Lots of good stuff already posted, much of which assumes you're familiar with the Lean Startup. If that isn't the case then it would be worth reading Ries, Blank and/or Maurya to tune in.
One big decision you'll have to face is whether this is what you want to do full time, or whether you want to continue doing what you're doing. I guess that establishing yourself in a relatively new job and pursuing your ACCA means your plate is pretty full at the moment. Most people in this position will choose to carry on with their careers, at least for the time being, and attempt to validate their idea at minimum cost, at least to the point where they can decide that it's really worth chucking their job in and applying to Y Combinator!
I think it's worth considering 2 perspectives - one longer term, one shorter term:
The longer term perspective is that you're unlikely to be funding this from your own resources. So at some point in the future you're going to be sitting in front of some investors making a pitch. Whether they're your local Chelmsford business angel network or some fancy VC on Sandhill Road, they will have some fundamental questions in their minds, including:
- Why this?
- Why now?
- Why you?
Why this? relates to the product, its market and the relationship between them. It's about whether there is a viable segment of the market that wants and will pay for the product,what alternatives they have, how easy they are to reach, what value you can create for them and what proportion of that value you can capture. The Lean context emphasizes the gathering of evidence to prove that you have in fact identified and can reach this initial niche, and have defined and can deliver a core set of features that they want and are willing to pay for.
Why now? is about market readiness. An element of this is to consider what changes have triggered the creation of the problem you're addressing, or the ability to solve it. An underlying thought is that if you're addressing a long standing problem and the technology to address it has been around for a while, then how come no-one has figured this out yet? That's not to say that no-one ever finds a $100 bill lying on the sidewalk, it's just that it's unusual.
It's very rare to find that there's no competition (apart from when there's no market!). Most of the time new products are better ways of solving old problems. So if you were developing an app to help people with anxiety, you might identify worry beads, Tetris and smoking as other ways in which people currently deal with this problem.
Why you?or more bluntly, 'why are you uniquely qualified to deliver on this opportunity?'. Good answers at various stages of development might include: having developed the foundations of a strong brand with a group of early adopters, having achieved early traction with a wider user base, having built a strong code base which puts you ahead in the race to deploy and capture eyeballs, having pulled together an effective and committed team. You might be (or become) a star coder, but my guess is that in this case a better answer is that you know everything about this market and why and how your solution is effective, which means researching in some depth how the problem you're solving manifests itself, different approaches that have been used to tackle it before and the pros and cons of currentalternatives.
In terms of a short term perspective, it's never too early to start talking to potential customers with a view to identifying the earliest adopters and working out how to reach them and define your MVP. I act as an external supervisor for an MSc course in Innovation & Entrepreneurship at Warwick (www2.warwick.ac.uk/fac/sci/wmg/education/wmgmasters/courses/innovation_and_entrepreneurship
) and one of my candidates this year completed an interesting dissertation on how 'minimum' an MVP can be. He found a difference between paper prototypes, wireframes and clickable mockups, but not as much as you might expect (although he was dealing with a technically qualified user base). So depending on your own preferences and abilities, you could think about using one of these techniques to facilitate early discussions with potential customers.
One final point - you'll need to be a bit careful if you're going to make any formal therapeutic claims for your product. Anything beyond recreational use could lead you into an ethical and/or statistical nightmare. To get a feel for some of the statistical issues 'How not to be Wrong' by Ellenberg is an accessible text.
Happy to connect and talk some more if you're interested