Thanks for responding and providing an answer. One of my attempts I had something similar;
DLookup("[msc_cmp_title]", "[tbl_ref_msc_cmp]", "[msc_cmp_ref]='' DLookUp("[gr_brand_ref]","tbl_ref_gear","[gr_ref]=Forms![frm_data_act_gear_sub]![act_hdr_gear_ftw_ref]")).
The reason have shown this is I don't have a separate text box from which I am deriving the initial reference number. How it works is that I have a drop down box from which the user can select the appropriate item. The selected item has its own unique text code (as there could be multiple incidents of the item) which is then posted to the field, as part of the item's dataset are other codes relating to a brand and brief description (note there are multiple brands and descriptions hence the separate db [tbl_ref_msc_cmp]). What I am attempting to do is when the user selects the item from the drop down rather than just show the reference I have a separate text field which then displays the brand and description for memo purposes.