Question: Could you advise - which is correct.

1.) We are pleased to offer you services of one of our developers.

Or

2.) We are pleased to offer you services of one of our developer.

Answer: 
"one of our" denotes more than one - so the correct word is "developers"

unless you mean to say 
"We are pleased to offer you services of our developer"
*We are pleased to offer you **the** services of one of our developer**s***.