Displaying a text value

Hello, 
I have 2 fields that record a reference number
[Case ID] (key field) and (numeric)
[Case ID from import] (contains legacy data from an old system).  (numeric)

I have created a Formula Rich Text field to combine and display these values in a large format at the top of a form.
 
"<span style='font-size:150%; font-weight:bold'>" &
If(not IsNull([Item ID From Import]), ToText([Item ID From Import]), ToText([Case ID]))
& "</span>"

The Item ID is pulling through but I must be missing something because if there is an [Item ID from Import] I see it but if not I just see a "0", not [Case ID]. 

I also tried this as 

"<span style='font-size:150%; font-weight:bold'>" &
If(not IsNull([Item ID From Import]), ([Item ID From Import]) &
If(isNull([Item ID from Import]), [Case ID]))
& "</span>"

But only the first part seems to work!

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Owen Morgan
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