I see that this "Dartnell-Delahoyde Tree GEDCOM" is really the file name of the Ancestry tree. It is NOT a link to a GEDCOM file.
You will need to contact the owner of this tree to get a GEDCOM file.
I do NOT have a UK subscription, so I will not be much more help.
But, if you have access to this tree, you should be able to use the Merge function from within FTM.
With the hint open and the correct person selected, click on the "Merge" icon.
What version of FTM are you running?