I found the answer on how to fix this but not yet why it happens.
I used the Import Wizard to create a Package & then looked at how it was constructed.
While stepping through the Import Wizard in SSMS, I found that 5 fields (starting with the 'Accnt_Number' field) had a value in the 'Size' field (11, 15, 15, 16 etc).
These fields were the ones that had to be converted to the DT_STR Data Type & the 'Code Page' field then autopopulates with '1252 (ANSI Latin 1)'
As soon as I had done this the error went away.
I'd like to know how the 'Size' field works & why it causes the need to do a data Conversion...
Should I have saved these fields as another data Type??