Nested IF, INDEX and MATCH in Google Sheets

I'm trying to return a value in Google sheets. This is done using an Index Match as follows, which does work:

=iferror(index(Data!B:B, match(B5339,Data!G:G,0)),"Not Found")

I'd now like to expand this, so that if this first test fails, try looking up that same data in another sheet....

=iferror(if(index(Data!B:B, match(B5340,Data!G:G,0),if(index(HeadOfficeLeads!B:B, match(B5340,HeadOfficeLeads!A:A,0))))),"Not found")

This outputs the fail msg of "Not Found". However, although the first test is indeed false, the second test is true (this second data set does in fact hold a match).

NB - the data containing this correct match on the 2nd sheet is created by a UNIQUE ( FILTER, FWIW....

For some reason, it doesnt look like the second IF statement is being run - and the whole thing doesnt work, giving the error "Wrong number of arguments".

I have a feeling the argument issue is that the first test doesnt have an "if false" clause - but believe the "IFERROR" parent should handle this? If not, where would I put the "if false cluase" for the IF's?