Borders taken from Image:2005UKElectionMap.svg
Colours are the same as used in that file, plus dark purple for UKUP.
See Image:Northern Ireland election seats 1997-2005.svg for corrisponding image without by-election.
I had to upload it a load of times because Wikipedia's SVG to PNG converter is really buggy, especially with font sizes.
Whilst it looked good locally, only after uploading would I find it was being mis-rendered :-(
Additionally, the strokes around the legend boxes aren't being drawn.

PNG version can be found at File:Northern Ireland election seats 1997-2005-by.png
No machine-readable source provided. Own work assumed (based on copyright claims).
No machine-readable author provided. Nickshanks assumed (based on copyright claims).