I am creating a parameter for my report where the Label is the employee name, and the value is their Staff No. This is fine if everyone only has one staff number, however I have a number of people who have multiple, and I need all the data to be pulled through when I select their name, so that means multiple values.
Is this possible and if so how, i have tried separating the numbers with commors but this makes the report fall over.

You should handle this in your query. Set up the parameter to accept an employee’s name, and then filter the data in the report based on a look-up of that employee:
The
@EmployeeNameparameter would be mapped to the SSRS parameter.