The coolant pump is a key part of the cooling system (https://www.vwgolfmk2.co.uk/clubforum/index.php?topic=24.msg28#msg28). The pump can be removed from the housing and replaced separately, however, it is easier to remove the pump and housing from the engine as a unit and separate them later, or just replace the whole unit.
(http://vnc.thewpp.ca/stuff/bentley/ep0niks.ctech.ca/vw/eva2/GE02/i21192.gif)
A new unit costs around £30 so it's hardly worth trying to service one
(http://i69.photobucket.com/albums/i61/eddypeck/GolfGti/2015-01-25144223_zpsf7b8c5e1.jpg)
When replacing the coolant pump on cars with power steering, the power steering pump and mounting brackets must be removed first.
On cars with air conditioning, replacement of the coolant pump will be considerably easier if the alternator and air conditioning compressor are removed. The air conditioning compressor should be removed from its mountings and set out of the way without loosening or removing the compressor hoses.
When replacing the pump take the opportunity to fit a new thermostat (https://www.vwgolfmk2.co.uk/clubforum/index.php?topic=25.new#new)and housing and always use new seals.