I see where you are coming from with this Gavin - but those sections relate to how to calculate the remaining social housing each Council has so that they can be allocated 'their share' of the total debt. This is not a reduction of the total debt with regards to RTB's homes etc - that money has not offset the debt as far as I can see.
For the debt to be reduced taking into account RTB receipts then somewhere you should be able to see an accounting entry for 2 Million times worth of around £60,000 at time coming off of the total build cost. Perhaps someone at the Treasury can confirm if that ever happened - but according to Clive Solely in 1993, it had not.
I'm sorry to hear about your grandmother by the way.