With respect Paul. Gareth isn't some half arsed newbie, you should maybe have checked out his portfolio before offering what turns out to be misplaced guidance.
Likewise with respect, whilst I may not have checked out his portfolio by his written words he
is coming across as a 'half arsed newbie' and I'm afraid that he has compounded it by this statement -
my OCF lighting knowledge is only theoretical. I KNOW what light does, just have never really given lighting with flash a proper go. Perhaps I should put time and effort into learning this next? Most stuff has been shot with available light.
Seriously what kind of a professional portrait photographer exists without having an extremely good knowledge of flash, plus all the kit to back it up?
Now I'm not here to personally attack the OP, I'm here to help and the reality is that sometimes people have to get real. I see countless newbie photographers with this dream of being a natural light photographer simply either because they don't know how to use and or can't afford decent flash units.
Fine if you prefer natural light. Fine if you intend to use it on a shoot, but as a professional if its not available, which it often isn't in this country, then you need backup plans a] b] and c]
What happens when you are booked by a client and you discuss the shoot and intend to shoot outside, but even though its nice and sunny it may be windy or too cold and the ladies never want to stand outside in that that. So you have to move inside. What do you do without flash? Limit yourself to high ISO window light shots? What then happens when the client tells you they have an absolute stunning room with no windows that they want to be photographed in, what do you do? Any professional would go to his car and pull out a few lighting heads.
This is all about being professional and operating in a professional manner. Its very, very common no matter how much you discuss a shoot to turn up on site and find things totally different or to find the client has been thinking about things over night and come up with a new idea and therefore you need the skills, knowledge and kit to go ahead in this new direction.
Why am I so hot on this? Because early on I got burnt several times by 'photographers' claiming this, claiming that only to find out to my cost how they couldn't actually do that or lots of other things when reasonably asked. Therefore I judge people on their words how they carry themselves as a photographer as much as their portfolio. Newbie photographers carry themselves in a totally different way to genuine professionals, its miles apart, so easy to spot by a professional.
OK, so getting back on track, with all the above - to the OP, yes its extremely vital that you go out get some quality flash heads Elinchrom, Bowens etc. at least 2, but budget to have at least 4 very shortly along with a selection of softboxes and reflectors etc. And also get yourself some bounces and flags and then learn how to use everything. Buy an old book on studio lighting. Forget any of the modern one, especially those with 'digital' in the title. Learn how to use and control light. Get a proper light meter. Start with inanimate objects and then progress to models, start with TFP models, then progress to paying for experienced models. Learn and study. Ask for and take and accept critique from professionals.
If you really have skill and really want to do it then you will achieve your goal.