Tier 1 General Extension: Advice needed on PEO process

[raghu0307]

Hi,

My Tier 1 visa expires on 16th March 2012 and I will be applying for extension first week of March. My application should be straight forward as I have all the required documents and supporting evidences. I have 2 dependants, my wife and daughter (born here). I wish to apply for all of us together.

My question is to do with one of the previous threads in this forum which spoke about the Croydon PEO process where the applicant was asked to wait for the decision due to a pending security check.
See here for details on that thread if required.

I would like to know:

1) What does the security check relate to?

2) What are the various circumstances due to which a decision may not be taken on the same day in a personal appointment? - (I know a couple like 'large volume of applicants' and the one above but would like more clarity)

3)Also can some one please brief the pros and cons of applying in person and by post

Thanks in advance.
Raghu