Sempre, charge them interest at the full amount (of your lump sum), at commercial rates of interest. It is none of their business what you intended to do with the money.
Under the old system, that worked, I received my lump sum on the last day of service.
The precedent is there and it costs peanuts to take them to the small claims court.
Regards,
Nige
BTW you can get 6.9% in a BS at the mo.