Removal of blank lines
With a bit of help I have solved this, but thought I would post the answer in case it helps someone else.

I have changed the expression to read:

=Fields!txtLabelSalutation.Value &amp; vbCRLF &amp; Fields!txtAddress1.Value &amp; vbCRLF &amp;
    IIF(
            Fields!txtAddress2.Value &lt;&gt; "", Fields!txtAddress2.Value &amp; vbCRLF, nothing) &amp;
    IIF (
            Fields!txtAddress3.Value &lt;&gt; "", Fields!txtAddress3.Value &amp; vbCRLF, nothing) &amp;
    IIF(        
            Fields!txtTown.Value &lt;&gt; "", Fields!txtTown.Value &amp; vbCRLF, nothing) &amp;
    IIF(
            Fields!txtCounty.Value &lt;&gt; "", Fields!txtCounty.Value &amp; vbCRLF, nothing) &amp;
    IIF(
            Fields!txtCountry.Value &lt;&gt; "United Kingdom", Fields!txtCountry.Value &amp; vbCRLF, nothing) &amp;
    IIF(
            Fields!txtPostcode.Value &lt;&gt; "", Fields!txtPostcode.Value &amp; vbCRLF, nothing)


My problem was where I was including the line break within the statement.