As mentioned previously, I was having some problems with not having the right operators in the Filter Criteria dialog of a DataForm Web Part. Although the field I was trying to filter by was a date, I was only being shown the options as if it was a string:

Well, proving that invention is 99% perspiration, I managed to find a solution. On the list on my customer’s system there was a Lookup column which referred to a list that no longer existed. Note the lack of List name for where it’s getting information:

When I removed that column from the list, I could filter correctly again:

This is despite the fact that the broken lookup column is not used either in the display on the DataForm webpart, or in it’s filter. Merely its presence is enough to screw things up. Note that this is a different solution to other ideas that have been suggested elsewhere.