There are leading tabs in some of the data.

For example, the [Mailing Address Line 1] that seems to be 'North Middlesex University Hospital NHS Trust' is actually that same string with a leading horizontal tab. The [Organisation Name] in the corresponding row in the other table does not have the leading tab, so they do not match.

You'll want to clean up all that before you can do any straightforward joining.

Cheers!