Glowworm 30SXI leaking from Blue cap?

My boiler is dripping from a Blue screw on cap, which is underneath a square-ish White Plastic Vessel, which is visible at the top rear behind the control panel, there is another pipe connected to this which goes outside. I have manged to tighten this cap up and the drip has reduced. What does this do and is it easy to replace (will my garage fill up with water if I remove the Cap)

Thanks Pete