What is the plus and minus screw inside front of boiler?

I have an worcester 15/20 oil heating boiler which needs servicing..and cleaning and as it is the christmas holidays decided to break down. I have cleaned which had been suggested by a heating engineer..(who does not perform services ).got a whole load of dust etc out of as we have recently moved in and find it has hardly ever been serviced... and re assembled and everything appears to be working fine .. however we do have a screw that we are not sure what it does and cant find any information about.... any advice please...

Roy
January 2011