If I have exactly the same in savings as I have on my offset mortgage, will I pay or receive any interest?
This is called 100% offset and theoretically it will mean you pay no interest on your mortgage and receive no interest on your savings.
In reality there will be slight anomalies as credit interest uses the actual number of days in the calendar month, while debit interest divides the year into 12 (so each ‘month’ is 30.4 days).
For example, in a 28-day February, the credit interest applied in March will be for 28 days while the debit charged will be for a twelfth of the year, ie 30.4 days. There will therefore be a difference.