Earliest possible when I can apply for ILR - URGENT HELP REQ

[sapphire73]

I have a situation whereby I got my work permit in 2008 and was granted visa to come into the UK in June'2008. I entered the country on 08th Oct'2008. Last year I changed my work permit to Tier-2 PBS visa which is valid until Aug'2015 because I got a gap of more than 90 days between visa stamp date and entry date.
I'll be completing my 5 years period in UK on 8th Oct'2013 and current visa is valid until'2015. I am thinking of making an PEO appointment for my ILR, I would like to ask can I make an application for ILR 28 days prior to completion of 5 years or do I need to wait for completion of 5 years. Please also let me know the clause/reference from UKBA which confirms the statement.
Any help would be greatly appreciated.