It's often said that getting an Indian visa is good preparation for your visit, as the confusing and often chaotic process is a good introduction to your time there... I think there's at least something to be said for that!
You would think that the place to look would be the Indian High Commission in London website, but that doesn't seem to contain the sorts of info you're probably used to finding. As best as I can tell, you need to apply online via a third party, then either post off your application, or deliver it to them in person. The website for that has all the links you'd expect, but doesn't always have the info you want on them!
Alternately, there are a number of agencies who'll handle it for you for an additional fee. Several of them offer a 3 day service for tourist visas, and quicker for business visas. There's possibly quite a bit to be said for paying one of those companies to sort it out for you...