This is an interesting one, and I would appreciate an Expert's take on this:
If the 12 months extension has been agreed in lieu of equity, surely, now, the property should be excluded from the arrangement and the RX1 restriction lifted. Or, would failure to complete the extension mean the property is still "at risk" (so the restriction remains, in case) ?
That apart, I would imagine that you could pre-empt matters and go to the creditors with an offer to sell, in order to clear the IVA, giving them an amount you feel comfortable to hand over, this would stipulate that you will not sell if the offer is rejected.
If the property is still considered to be an asset (per my opening question), which I suspect it will be, and you sell regardless, they will take such equity as is required to clear the total outstanding, original, debt, plus fees and possible statutory interest.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014