>Cetin, I think I have got to the bottom of it
>I did have a filter on the file (set filter to cusref = thisform.objMemvar.Cusref) which is set in the calling screen.
>
>In the called screen,presumably this filter is still on but it cannot find ThisForm.. now because the ThisForm is now the calling Parent.
>
>(Maybe the reason it did not error out on a Seek 'abc' was because the seek got an EOF and it did not trigger the filter ??
>
>Anyway, this poses another problem. Is there a way of maintaining a filter across called screens (I have set up called screens so they share the DE)

In the calling form, set a variable to the value you need, then set filter to the literal value of it, so it won't try to re-evaluate it each time:

lcVal=thisform.objMemvar.Cusref
set filter to cusRef= "&lcVal"

If you look at set("filter") you should see it set to a literal string, not to an object property. Nice thing with literal strings is they never get out of scope :).