Question: why you use distinct and also * ? What you want to obtain from this query?
If I don't know the DDL/structure of your tables , I have to guess
CREATE NONCLUSTERED INDEX IX_NC_Project_ProjectNo ON Project(ProjectNo);
CREATE NONCLUSTERED INDEX IX_NC_Labour_ProjectNo ON Labour(ProjectNo);
where: IX_NC_Project_ProjectNo and IX_NC_Labour_ProjectNo are the name of the index.This could be any unique name.
pls check BOL (BooksOnLine)
inner join labour
ON ltrim(ltrim(project.project_no)) = ltrim(rtrim(labour.project_no))