FATCA - mortgage

does a mortgage with a UK bank have to be reported for FATCA? is that a 'debt interest' in a foreign financial institutio?

A mortgage is not a debt interest, it's a loan. If you have an endowment mortgage that provides any cash surrender value, then that policy would need to be disclosed in part II. 

Debit interests include debt securities. An example would be an individual bond. You loan the U.K government money, in return they pay you interest. NS&I would be an example. High St banks offer similar accounts. 

A mortgage is not a debt interest, it's a loan. If you have an endowment mortgage that provides any cash surrender value, then that policy would need to be disclosed in part II. 

....and an offset mortgage would be reportable as you have the associated account

It makes a big difference, so let me be sure of this. for an offset mortgage, I count only the savings/'offset' facility and not the mortgage debt?

Um... but if it's associated, then it's a linked account.  A separate account from the mortgage debt?

only 1 account number for both

only 1 account number for both

If you have a credit balance against the mortgage debt all in one account, then this seems to me more like a current account mortgage.