The citation you have is to the English Reports. It is a reprint set that gathered reports from various early nominative reports (nominative reports are named after the reporter who compiled them). It covers selected decisions from 1220-1873.

Are you familiar with legal citations generally? The citation contains the case name, decision date (1560), volume containing the case (75), the abbreviated name of the set in which it’s published (English Reports) and the first page on which the case appears (305)

The English Reports are available in page image format at commonlii. It is also available via the HeinOnline database, if you have access to a law library.

You mention that you have several cases to read. If they are all ER citations, they should be available at commonlii. If you have other citations, the tools below may be of help:

Cardiff index to legal abbreviations: allows you to enter the abbreviation and identify which reporter includes your case

Yale guide to finding English Cases information and links to sources for case finding

Note that even if you have nominative citations, the case might still appear in the English Reports. Commonlii allows you to look up cases by the case name, while HeinOnline has a conversion table.

If you’re still having trouble locating sources, I’d recommend contacting a law library, particularly an academic law school library for additional help.

Thank you, reached out to a uni law library and they also explained basically exactly this. Found the case though :)