ILR under ICT required employer documents

[KTHM]

Hi All, first time user. I hear a lot of praising for this site. so appreciate any help.

I am on ICT Tier 2 Visa. My wife and kids are my dependants.

I spent 5.5 years in the UK. I didn't apply yet due to issues with extensive travel for work ending with couple days beyond the 180 days for one of the years. Hence ICT Visa extended for another 3 years.

There's a window for me to apply with confirmed issue of the 180 days limit for each year from the date of submitting the application being resolved (all 5 years since that date each year having less than the total 180 days).

I have full file ready for me and my dependants, and of course will gather the bank statements and pay slips for the last 3 months before applying. 

Since the company submitted already a COS for me covering next 3 years, It seems I may be applying out of pocket.

My question is what paperwork do I need from my employer? I assume company headed lettersupporting my travel. anything else? Do I need a letterstating that I am employed by my company? or the fact that I am on COS for the next 3 years is enough? if that letter is needed, is it just a letter stating I am employed with them or does it need to be a newCOS?