More weeds than lawn...help

[NickRobinsonsloveslave]

Can anyone recommend a really fast acting weed killer as my lawn is absolutely covered in weeds. It also has bare patches and is clay soil based.
Most weedkillers seem to take 4 - 6 weeks to work.
Anyone know of anything that works faster?

I use these people. I've got a garden that's 80 by 18 foot, and they charge me £16 every three months to treat it. The lawn had loads of weeds and dandylions in it when we moved here, and within 2 treatments, they've gone. I still use them, as to buy the weedkiller would cost me the same as them doing it, but even if you just had them for a couple of visits, it might get it under control enough for you to tackle yourself.