Tier 1 extension within 28days of exxpiry

Hi All

I applied for tier 1 extension within 28 days of expiry of my existing leave. but still my leave was granted from the date of processing rather than being from the end of my last leave. So it now leaves me with a gap of 4 days..

I mailed ukba for that and i got response saying

"I have checked your current leave to remain dates for yourself and your spouse and they are correct.

Renewals are made from the date they are worked on by the caseworker, in your case, 16 April 2013."

Can someone please guide me to a document where it says that we must apply with in 28days only else there can be a gap in time to qualifying period.

Any help would be appreciated.

Thanks
M.