Well, that's taken me over 2½ hours to solve but I think I have MY solution. There appear to be various 'options' out there to finding the location of a parameter that you can't delete from your report. 1) Change its data type - that will cause an error; nope. 2) Export the report AS a Report Definition & have a look; nope. 3) Save the Sub-Report AS & reimport it - not quite but it put me on the right lines 
My solution to my problem:- In Design View, Right click the Sub Report(s in turn), Change Subreport Links... and select the parameter you need to remove, from the list of 'Field(s) to link to: and then < button it out of the list.
Only after removing from ALL the Sub Report Links was I relieved to see that pesky green tick disappear from the Parameter Fields list.
If this helps even one person I'll be happy.
Des.
P.S. If there was a better way of finding out, please let me know.
My solution to my problem:- In Design View, Right click the Sub Report(s in turn), Change Subreport Links... and select the parameter you need to remove, from the list of 'Field(s) to link to: and then < button it out of the list.
Only after removing from ALL the Sub Report Links was I relieved to see that pesky green tick disappear from the Parameter Fields list.
If this helps even one person I'll be happy.
Des.
P.S. If there was a better way of finding out, please let me know.