Try using save/send records as snap shot link. You can open the file produced by this "save" and it should reproduce the found set you had at the time you selected this option. This file could then be inserted into a container field to keep it with your database.
Or you can save a list of Primary Key values in a field and use it via a Go To Related Records step to recreate your found set of records. In FileMaker 13, the new "list of" summary field can make this much simpler to set up than in earlier versions, but you'll still need to set up a system to recreate the found set from this list.
You can do this by scripting. Write a script that loops and collects all the specific primary keys of your set of records. Save this and give it a name. Then you can retrieve it later. I'm heading out the door right now, if you don't get a complete answer by the time I get back, Ill post the process & script
Many thanks. Having more or less got a working solution I had resisted further efforts to get to really know the programme - I had avoided scripts for example altogether. The forum is great f can move forward again now. Many thanks both.