@ScottShearer Thanks for your reply. I've been trying to solve this for a while now.
The SharePoint list contains all the fields that are recorded on the MS Form, plus some more that are entered later.
These include: forename, surname, date of birth, email address and other basic demographics.
Ideally, if I could get the Flow to update once finding a match for the surname and date of birth, or the email address, that would be perfect.
Do you know of any examples that I could try and apply to my own scenario? Thanks very much for your help.