Stumbled across this post, detailing the process of how the author creates (and filters) startup ideas. Which is pretty interesting, but not of direct relevance to me as I have one startup idea, and just want to make that work, not create more!
However of even greater interest was the planning worksheet that he uses. Fortunately. a commenter pointed me to the Business Model Generation website, which created the worksheet.
Their resources include a hosted shared worksheet, a free PDF download (available here), and an iPad app. The iPad app is a bit steep (£20), but i eventually bit the bullet and bought it, and I’ll let you know how I get on.