Column Difference From Same Table

I have a table that contains a significant amount of data. The columns are as follows;

ID, WeekNo, Ref, Cost

What I want a query to do is only show me differences in Cost value where the Ref is the same

For example

ID, WeekNo, Ref, Cost
1, 1, ABC, 123
2, 1, DEF, 456
3, 2, ABC, 123
4, 2, DEF, 789

The query I would like, would only show me the result of Rows 2 & 4 because the value of cost is different between the weeks.

At the moment I have two queries, One that pulls out the last weeks value (WeekNo 1), and the other that pulls out the same information but for the comparison week (WeekNo 2). In the third query I created I linked the two columns from the Ref fields, but could not work the logic to only show me ones where the differences in cost were displayed.

Thanks in advance of the resolution to this. I know it is basic query work, but just have a logical block in getting it to work.

Regards