the soonest you can apply is upto 28 days before your current visa expires
so 16-08-2013 - 28 days is 19th july.
and the latest date is that they should receive it before your visa expires on the 16th Aug

if you are using the settlement checking service i think they take the payment when they checked your app and its all ok,

i think its £1,051 and also you need to pay for using the settlement checking service.