Refactoring a backend feature
Developed a backend system
Technical owner and architect of corporate bank reconciliation system, had been built by contract staff to meet a deadline.  System was majorly slow, brittle, unreliable, averaging 10 full days effort support per month end (each a severity 1 incident as failure to report by day 7 would allow FSA to stop bank trading), and was being pushed to be junked.  Redesigned major parts of system and led team to rebuild it, removing external dependencies, increasing performance majorly (day 2 load went from 16 hrs -> 45 mins), reducing support needs to near zero, added capabilities (could now load extracts from a 5 day period), and took system to point where, in merger with LBG, it was preferred choice for the merged corporate banks.