[Risky]

Thanks people for your help .. your suggestions greatly helped, but it wasn't the full story. Here's what else is required if it helps anyone else.

If you just edit help.php, and the customer accidentally still omits any of the fields you have left as mandatory, there is code in the help/contactus.tpl that also checks and marks fields on the Contact Us form that are mandatory and it will still find the omissions.

So, you need to edit the tpl file to remove or comment out these checks on the fields that aren't mandatory, PLUS you need to remove the red asterisks that mark those same fields as being mandatory (for the customer to see). I simply replaced the "*" characters with "&nbsp" (which will make it easier if I ever want to reinstate some/all of my changes later on).

Now, the form looks and works exactly like I wanted, with only a few mandatory fields.