Select Record Field Or ID From Subform

Okay - have hunted the web for the answers to this query on and off for months. Thought I had something but it didn't work.

Problem - basic form with subform of a 'select * from table' dataset. So far I've added one textbox. I'd like this textbox to show the recordnumber selected in the subform or the KeyID of the record selected in the subform. This is so I can populate other textboxes with details of that record for editing and then control/validate data entry.

Have tried: form timer, form load and some version of seltop (not bothered about selheight at this stage) - sort of worked but not properly - at least it gave 'an' answer but not always the correct one and you had to click in and out of the subform to generate an answer.

Private Sub MySubform_MouseUp(Button As Integer, Shift As Integer, X As Single, Y As Single)
Text4.Value = MySubform.Form.SelTop
End Sub

Private Sub Form_Load()
Me.TimerInterval = 10000
End Sub

Private Sub Form_Timer()
Text4.Value = MySubform.Form.SelTop
End Sub

I think the _MouseUp sub isn't working as its not a recognised event for a subform.

I started looking at various DisplayRowID, SelRecord functions (in a separate module) run from a command button in the form footer but that's not really the right sort of thing and the referencing to the subform is a bit iffy.

I'd be grateful if anyone could advise on, even complicated, code.
Thanks,
Lez