They are allowed to hold onto your lump sum for 30 days to make certain that you don't owe any money (LSAP, recovery charges etc)
They held mine for the full 30 then about a month late told me I owed 90 odd quid on LSAP. I ignored this (as I was pretty sure they were wrong) then another month later they thanked me for discharging the debt and released any "charge" they had on my house.
It seems that they work in 30 day steps but I was not refunded any interest on my held over lump sum and if this is done to everyone then it would add up.
Been out 9 months now and have just got my tax sorted.