This information supplied courtesy of Wizard original post here

We are going to Croydon on saturday morning to hand deliver our application for the "Right to remain" and to be on hand for any questions arising. We have to pay the british government £950 for this pleasure. Without boring you with our personal details, we have to do it in one day because my wife has to be back in Nanning this coming Tuesday (and i follow in a few weeks ) anyway, the note of caution is this:
The 2 year visa starts on the date of issue in china, the date of residence starts when she walks through immigration at Heathrow (or where ever). in our case there is a 20 day difference (I:E: issued on the 8th , arrived on the 28th). but you are not allowed to apply for the "Right to remain" or "citzenship" (after passing the citizenship test etc) before 28 days before the residency ends. of course you cannot apply if your visa has expired and therefore illegal. so that in our case we have a 8 day window to appy (either by post, arriving in the 8 day window, or by hand at extra cost), please note that if your visa expires while your application is being processed you are still ok, unless the application fails then you must leave immediatlly.
The point is lads please tell your wives that when the visa is issued, get on the plane as soon as possible. if she gets on the plane 29 days after issue you will not be able to apply for citzenship 2 years further down the road unless you apply (and pay £500) for a 6 month extension
ok thats it