I just recently heard about these guys myself.  Rumor has it that “The Increment” as it was known was a small, loosely organized team of active duty British SAS soldiers who worked with and supported MI6 intelligence operations.  Today, it seems that they have been more formally organized as E-Squadron of the SAS.  More than likely they provide low visibility operations and options for intelligence agents who may have to operate in semi-permissive environments.  So far, our best information about them comes from the BBC article below.

From the BBC:

“The existence of E Squadron is well known within the special forces community but has not hitherto been discussed publicly. It was formed five years ago to work closely with the intelligence service MI6, and is mainly involved in missions where maximum discretion is required, say Whitehall insiders.

Its role as a small, handpicked force operating with MI6 makes it the modern-day successor to the shadowy cell sometimes referred to as the Increment.