Don't think you are being thick at all and it's actually nice to have a sensible conversation about this.
Explain to me how you would apply a NSNL clause to the current deal on the table. Also I don't totally understand what you mean by pre requirement on the sale.
Probably me going round in circles here so I won’t keep going back and forth - but if the council accept the £2.05m * offer from CDNL - justified by cash up front and no breaking lease complications (Therefore preferable compared with the other offer on the table) why can’t they put a clause into the agreement - a simple condition of sale.
* Typo on amount corrected where I originally put £3 million