I HATE seeing "distraught and frustrated Welsh lasses", so I will do my part for chivalry and international relations, if you will accept assistance from a Yank knight in somewhat tarnished armour....

I would like to clarify what you are asking for.
1. Column "A" of the "Destination" sheet has ALL possible values from the "Source" sheet.

2. Column "A" of the "Source" sheet will NOT have all possible "Destination" values

3. Copy a value from the "Source" sheet to the "Destination" sheet based on the value in column "A"

4. Will there be more than 1 instance on the "Source" sheet of any given value (will there be more than 1 occurence of a value?) will these multiple occurrences have a corresponding value on the "Destination" sheet?