If you just want "FIRSTNAME LASTNAME" from UserT your DLookup would be
=DLookup("FirstName & "" "" & LastName", "UserT", "UserID=" & UserID)

If you're trying to look up the first/last that matches the contact record. If you're looking for the first/last of the userid of the person logged on, then it would be:

=DLookup("FirstName & "" "" & LastName", "UserT", "UserID = " & Forms!MainMenuF!UserID)

DLookup
Concatenation

It is not recommended using DLookup in a Continuous Form. The function would have to run for every record in your recordset and if you have a lot if records can run really slow. If your backend is on , for example, SQL Server then it is going to run extremely slow.

I would build a Query for this with a Calculated Field for FullName.

Thank you Donald, Alex and Kevin for taking the time to reply (wow that was quick!)
Donald, I couldn't get it to work but I think it might be down to something else I've not yet spotted on my side, either tweaking the code or something else in the database. I will let you know when it does but for now I'm going to take Kevin's advice and switch back to using a query instead, so the database runs efficiently. Thanks for pointing that out Kevin. Thank you all again for your contributions - it is most appreciated :)

Hi again. So, as suggested, I created a calculated field in a query (for the continuous subform with the contact history log) and this worked: it populated the Fullname field on the continuous form with the current user's Fullname.
However, after a different user logs on, and if the contact history subform is reopened, all the Fullname fields of all the previously logged records are overwritten with the current user's Fullname.
How do I get it to leave the previous records alone and only add the current user Fullname to the current (new) selected record on the continuous subform when the form loads.
For info: I'm actually using 'LoggedBy' as field name instead of Fullname; so then the calculated field in the query for the continuous subform is:
LoggedBy: DLookUp("FirstName & "" "" & LastName","UserT","UserID=" & [Forms]![MainMenuF2]![txtUserID])

Why do you think it's changing?

I think it makes sense as in that he query is commanding the LoggedBy field on the continuous form to search the UserID of the current logged on user and update to their UserID as soon as the form loads. This outputs the User's concatenated fullname which is fine for the current record.
It's evident that I've not done something to instruct it to only affect the current (new record) with the current UserID.
I've trawled through lots of code in the 599cd forums, searched the topic several different ways. Tried Before Update, also tried editing the allow edits option in the properties section for that field. Considered but unsure how to use the On Current event in VBA but come up with nothing yet.

You'd need to have a Field in your Table called UserID that you set when the Record is created.

You could then use a Query to pull in the First/Last from your UserT.

Okay, that has worked! Thank you Alex.