Great news!
Thanks to leonowski the problem seems to have been solved. Image may be NSFW.
Clik here to view.
If you put xls into the "specify file types/Additionally index all files with these extensions" box it apparently forces X1 to use offfilt.dll rather than the stellent filter. And it works! I cleared the index files and re-indexed and now I can find numbers in the .xls files with the cells formatted as
General
Text
Percent
Number
Custom
Currency
Only thing it doesn't do is highlight the search term in the spreadsheet in the preview pane but I can live with that.
Looking at the Indexing status for all my indexed files, if content is indexed it says "Stellent OK" apart from .xls files where it says "IFilter OK".
This is using X1 version 6.6.5 (build 3926) and Offilt.dll version 2006.0.5732.0. MS Office 2003 SP2, Windows XP Pro SP3
Thanks to leonowski the problem seems to have been solved. Image may be NSFW.
Clik here to view.

If you put xls into the "specify file types/Additionally index all files with these extensions" box it apparently forces X1 to use offfilt.dll rather than the stellent filter. And it works! I cleared the index files and re-indexed and now I can find numbers in the .xls files with the cells formatted as
General
Text
Percent
Number
Custom
Currency
Only thing it doesn't do is highlight the search term in the spreadsheet in the preview pane but I can live with that.
Looking at the Indexing status for all my indexed files, if content is indexed it says "Stellent OK" apart from .xls files where it says "IFilter OK".
This is using X1 version 6.6.5 (build 3926) and Offilt.dll version 2006.0.5732.0. MS Office 2003 SP2, Windows XP Pro SP3