I have just opened LibreOffice Calc, and the spreadsheet appears minimised with Cell DM showing in the top right corner, and perhaps 250 lines, the line entry being unreadable

I am running Version: in Calc.Build ID: 420m0(Build:3) English
The OS is the updated version of MInt 17, Cinnamon 64 bit English

Is it possible to provide a screen shot?

Check the Zoom slider in the bottom right corner (Status Bar) - if your zoom % is significantly less than 100% you will see a very large portion of the sheet.