Not cancelling the DD was a wise move, as VM do have 'previous' for adding late payment fees in similar situations and eventually declaring the poor customer as a 'debtor' and handing the whole lot onto a debt collection agency!
Probably too late now, but the letter should probably have requested an additional £20-£50 in compensation for the inconvenience and distress that has been caused to a 'vulnerable customer'*. Incidentally was the letter sent by recorded (signed for) delivery? Now the reason I ask is that there is a greater than zero chance that you or you brother-in-law will hear nothing more, call VM and their reply will be along the lines of 'letter, what letter, we've had no letter', who are you again?'
I would make yourself acquainted with the formal VM complaint procedure and if this goes nowhere, or they offer something which you all consider to be substandard as way of recompense, then also research how to raise a compliant with the industry ombudsman service, which is the independent arbitration service. In this event, I do feel, you would be justified in asking them to order VM to credit your father with an additional £100 in reflection of the added inconvenience and stress.
* always a useful phrase to drop in to formal complaints or conversations.
