Thank you all for your suggestions. Using Patrice's prod to record a macro, I found that my original code was causing the problem. To freeze the rows at Row and Col I was using -
xlWorksheet.Application.ActiveWindow.SplitRow = Row;
xlWorksheet.Application.ActiveWindow.SplitColumn = Col;
This 'SplitRow/SplitColumn' was apparently achieving the desired freezing, but was not easily reversible. The recorded macro showed that all that was required was to highlight the row below and the column to the right, and, then FreezePanes. So this code worked -
Range rng = xlWorksheet.Cells[Row+1, Col+1];
rng.Select();
xlWorksheet.Application.ActiveWindow.FreezePanes = true;
Then to clear the sheet of frozen panes, formatting and data the code is simply -
xlWorksheet.Application.ActiveWindow.FreezePanes = false;
xlWorksheet.UsedRange.ClearFormats();
xlWorksheet.UsedRange.ClearContents();
Additionally, replacing the ClearContents with
xlWorksheet.UsedRange.Delete(XlDeleteShiftDirection.xlShiftToLeft);
restores the column widths as well.
Thanks again