Mortgage on credit report

I've been discharged since December and the building society have finally repossessed the house, taking possession at the beginning of April.
On my Credit report it is still showing as an open account with a large balance. How soon should I wait before sending them the clean-up letter from this forum, which worked so well with other creditors.
I presume they will eventually mark it as satisfied or partially satisfied with the appropriate dates or is it different with house repossession?