I’ve found a marvellous page on the internet, providing background material for a business course titled “MFIN 7011: Credit Risk” offered by the University of Hong Kong School of Business, taught by Dragon Yongjun Tang.
The course schedule and downloads provide some good spreadsheets for computations on the Merton Model – although the “Loffler and Posch CDS Spreadsheet” does not appear to complete, missing the function “Yearfrac()”.
Update: The “Yearfrac()” function is part of the Analysis ToolPak. In Excel, under “Tools | Add-Ins” ensure that “Analysis ToolPak – VBA” has been selected.