Just an update. My council has a 'drop in' service so I went and spoke to a planning officer who said (verbally - written advice is a paid for pre-app!) that this would come under permitted development because it is not an access to the building or attached to the house (i.e. a means of getting around the house from one floor to another).
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!