Try including the ".Form" bit to distinguish between the subform control and
the form in that control:
[Forms]![FM_MainDetails]![FMS_Boat_subform].Form![TM_JobDetails_subform].LinkMasterFields
= "jobno"
Explanation:
Referring to Controls on a Subform
at:
http://allenbrowne.com/casu-04.html
If that still doesn't work, open the main form in design view, and check the
Name property of the subform control. The name can be different than the
name of the form loaded into it (its SourceObject.)

Signature
Allen Browne - Microsoft MVP. Perth, Western Australia
Tips for Access users - http://allenbrowne.com/tips.html
Reply to group, rather than allenbrowne at mvps dot org.
> Hi
>
[quoted text clipped - 19 lines]
>
> Stapes
Stapes - 28 Feb 2007 09:29 GMT
Hi
I tried that solution - putting .Form. between Form & subform. It
didn't work. I don't think the problem is with referencing the
LinkChild & LinkMaster fields, because the values are ending up in the
right place. What it is not finding is "jobno" which is a control on
FM_MainDetails. I moved my hidden control to the FMS_Boat_subform -
and it worked !!
Stapes