Interesting, as Brendan said.
There is a myth around which suggests that a draft has a status similar to cash. Not true! A draft is merely a cheque drawn on a bank, and has the same status as any cheque issued in the name of the bank - and in these days, the banks name counts for a lot less than in the past..
A related issue is the prevalence of frauds perpetrated using fraudelent or forged drafts.
I'm assuming that the draft was issued by a bank other than your bank. I'm also assuming that the redemption figure quoted was based upon the loan being paid off on the Friday.
If the bank had paid the draft directly into your mortgage account, and it subsequently was found to be worthless, the bank would likely have considerable difficulty recovering the funds from you. When they placed it in the current account, they could simply debit it to your current account if it bounced. So the bank was being protective by holding it in the current account first.
On the question of interest, it is normal practice for banks to charge interest on uncleared cheques (including drafts) until the day on which the bank gets funds. In your case, if the cheque reached the bank after their cut-off time on a Friday, then the cheque should be included in their clearing of Monday, which would be exchanged with the drawer bank on the Tuesday, and your bank would then receive payment on the Tuesday. So, if they had done things correctly, they might expect to charge 4 days - Fri, Sat, Sun, and Mon.
You could quibble with them about one day. But, don't forget that they still carried a risk associated with clearing that cheque for a further couple of days.
The sooner that the banks and the legal profession move to electronic settlement of this type of transaction the better.