Thanks for your replies guys. They are very useful insights and sound advice.
I'm under no illusions about how difficult a step it is to set up as a business, and 100% realise that it's not going to be a case of magically turning my hobby into a career. But Gary you're right that if you don't jump you will nevery fly...
I'm actually thinking of starting out by reducing the hours at my current full time job to give me the time to start building a decent portfolio and client base. It's the transistion period I'm most concerned about, and i guess whether it's possible to make enough money.
My aim is to combine a number of my skills / contacts into one package that should give me the competitive edge over others in my area. My wife runs a wedding stationery business, her best mate is a beautician, I am a IT professional specializing in internet applications.
So I "should" be able to get a few weddings under my belt, along with a few "makeover" photo shoots, and offer a package for small businesses to have pictures taken and create a website for them. This approach "should" mean that I'm not putting all my eggs in one basket and will "hopefully" let me assess the market in each of these areas while building a decent portfolio, and seeing what suits my style / skills best. In parallel I'm also planning on targeting one or two genres of magazine to try and build a "published" portfolio, and NUJ membership (is it worth it?).
But it all takes time, and I suspect most of that time will be spent doing research, sales and marketing... But hey, if it works out.... I'll be very glad not to be stuck in an office 9-5 every day.