Q:

How to debug An SSRS error - "Input String was not in Correct Format"

I know I've beem askimg alot, I am learning daly! 

The offending expression is:
=IIf(Parameters!ResponseRange.Value = 3, Nothing, 
    IIf(Parameters!ResponseRange.Value is Nothing, Nothing,
        Parameters!ResponseRange.Value))

It's spec. because of this part - Parameters!ResponseRange.Value is Nothing, Nothing, 
As for the Parameters, here's a screenshot of how they look - the Parameter is a "Text" data type:

A:

The order of the IIf() checks in that expression are odd; generally you want to handle the "success" or the non-null cases first. Also, you can combine the two checks into one IIf():
=IIf(Not IsNothing(Parameters!ResponseRange.Value) AndAlso 
    Parameters!ResponseRange.Value <> 3, Parameters!ResponseRange.Value, Nothing)

Note that I substituted the IsNothing() function for the Is Nothing expression. Other than that, the expression itself is valid. What is the data type for the ResponseRange parameter?  I'm deducing that it is an Integer via the comparison to 3. Do you have it marked that it can be null? Primitive data types like Integer can't be set to Nothing like you're attempting if you haven't made them nullable.
You may need to set a default value of 0 or -1 instead of Nothing and handle that subsequently in your query.

