you might find this site useful
http://www.parktool.com/repair_help/FAQindex.shtml

you'll need a tool like this:
to pull the cranks.
and this tool:
to remove the old BB and install the new one.

if you get a new crank, you'll likely need to get new BB as well. different cranks use different BB spindle lengths (and types) to achieve proper chainline. If you go with ultegra cranks you'll need a 109.5 mm Octalink V1 BB. Sugino 75 Track cranks use a 109mm square taper BB.
Your Langster should have a 68mm BB shell with British threading.
a couple of good crank/spindle length info sources:
http://www.sheldonbrown.com/bbsize.html for road cranks
http://businesscycles.com/tr-refspec.htm#cranks for track cranks.