do i have to re-install an older version of xampp
You must not do this, but you can.
can i just get hold of an older version of mySQL,
That's possible. Just install in another dir and start this MySQL server and not the one from XAMPP.
and if so, do i just over-write the version that is in the xampp folder at the moment.
That's also possible. But be carefull of the MySQL system tables. (they have a different layout between the MySQL versions).
The best way is to correct the SQL. Then your querys are working with all MySQL versions.