I think Tableau has a little problem with the precision beyond seconds.
If you don't mind losing the .0000000 after the 18, the following calculation works:
DATETIME( str(day([Date])) + "/" + str(month([Date])) + "/" + str(year([Date])) + " " + left([TimeString] ,8))
Hope that helps,
Thank you so, so much for your response and I got it to work once I'd swapped str(day([Date])) and str(month([Date])) as for some reason it was bringing the month as the day and the day as the month. But as soon as I'd done that your script worked perfectly.
Many thanks for helping me with this, I'm very grateful.