Shared ownership sale and stamp duty

[rc66]

I am currently in the process of selling my 50% share in a housing association flat. I was unable to find a buyer for the 50% share so I am selling the property on the open market. This means that immediatly prior to the sale I will staircase to 100% ownership and the purchaser will purchase 100% of the lease from me. The full price we have agree for the flat is £100,000. The buyer's solicitor has noted that when the shared lease was sold to the original purchaser (from whom I later bought the property) they did not pay stamp duty. This was in 1992 when the threshold for stamp duty was £30,000 and the 50% share was bought for £20,000 meaning the full market value was £40,000. The buyer's solicitor seems to think that I am going to have to pay some stamp duty which is a bit of a shock as I thought I was selling the property and had not factored in this expense. Does anyone have any idea of whether it is correct that I will have to pay stamp duty and if so, will it be 1% of the original £40,000 purcahse price or 1% of the value now, is £100,000 (which seems odd as the threahold for paying stamp duty is now higher). Thanks for any advice.