I now have a solution to this, which did eventually come from Progresso, albeit via a circuitous route:
You can add a previous year's assessment template to a current year's assessment screen, and can then access fields from both years on a single report.
To do this you go to
Assessments > Assessment Settings > Copy Assessment Screens
Select 2017/2018 to copy from and 2018/2019 to copy to
You then tick the assessments that you want to copy and click on Copy Selected.
Once the assessments have been copied into the new academic year you will need to edit them to show assessment data from the previous year as well as the current year.
To do this you open the Screen and Edit > in Properties you add the previous year's assessment template in Subject Teacher Entry Templates
If you then click on Step 3 you can tick or untick to select the columns you want to appear in your screen.
