Thanks a lot for the help guys...
However I think I did a pretty poor job explaining what I am trying to do.
I have a query that pulls from several tables. The result of this table is used in a subform as a list of items which primarily will be used for visual analysis.
One of the fields, [Owner] is empty until a particular item is used in an auction and then the name of the winner of that particular auction populates this field (there are more items than there will be auctions so not all items will have this field populated).
This subform - used for analysis - will show all records, who owns them (if anyone) and several other parameters.
If I put "Is Null" (without the quotes) as a criteria for the [Owner] field, the query returns only those items which have no owner; this is perfect. I now have a list of items that are available.
However I would like to toggle between the available items and all items including available and non-available.
Hence the criterion's logic I am looking for is, "If the checkbox 'Filter Unavailable' is off, then leave the criteria empty. Else if the filter is on, make the criteria "Is Null" - show only those items with nothing in this field".
I have done this with other criteria in other fields using this logic but where the criteria required (on tick of the tickbox) is a simple letter to be searched for:
Works perfectly since the filter I want is the letter G and not a command, but I need a version of this with Is Null instead of "G".
I hope this explains it a little better