Hmmm. I'm not one of the people you're really looking for here, but I could take a stab at reasons why I haven't been beating a path to your door...
a) I haven't really heard of you. A few words here and there over the years, but how much do you get out there and tell the geek community you exist? I'm guessing it's specific to a few specific tech areas - which I haven't seen.
b) You're in London, you talk about "working hard" and being in your "Clerkenwell office". I've worked round there before, but really? I want a company who's more about working smart - and not in one place all the time. How much do you really need to physically see me? Commuting, plus a little bit of a tiny hint of "macho hard work culture" (probably unfair, but...) is enough for me to take a place off my list. It's a sellers market, generally.
c) Also... money. London is expensive. Everyone knows London is expensive (and they're right). You don't say anything about money - no hints, etc. My reaction (being picky) is that it probably isn't enough to make it worth my while (and also that you may try and make me the lowest offer you think you can - I'm assuming you don't have open pay scales).
d) Effort. You talk about your tough hiring - multiple rounds, tests, a day working with you (you don't say whether you'll pay me for this...) Why would I do this? If I'm smart, capable, good at what I do, I probably know people who want to work with me already - why jump through these hoops for a possibly good job then?
These may be unfair points - I hope not, but perhaps - but finding good people right now is about attracting them, not throwing up barriers or making them work for it.