If your wardrobe really does need an overhaul, think about buying just a jacket. If you've got a smart dress, or a pair of black trousers, a tailored jacket should do the trick. You can buy them on the high street for as little as £50 and you'll wear it with everything (Topshop does an excellent range of jackets).
Topshop 3/4 sleeve blazer, £55
Just make sure that your outfit isn't the wrong side of dressy. If you look like you could go out to a club straight after your interview, you're not dressed appropriately. But if you're going for a less formal meeting, suggest meeting in a cafe so you can dress down slightly. It might even be worth saving that jacket in case you get a second interview.