According to the drawings in the manual (https://www.worcester-bosch.co.uk/support/document/download/release/8716115167/12651) it's the automatic air vent (See page 8, item 27).
Did a quick Google search for
https://www.diynot.com/diy/threads/worcester-bosch-28i-leak-from-auto-air-valve.29644/
https://www.youtube.com/watch?v=FYA9lFT5_cM
http://community.screwfix.com/threads/worcester-24i-combi-problem.73383/
Other results out there for a search for "worcester greenstar 24i leaking air vent"
acp
March 2017