How many properties are worth less than �120,000 now? How many of them are in non Labour areas?
Now what difference will it make anyway? A saving of �1000 on a transaction of �100,000 is hardly noticeable, and the seller will probalbly bump the price up anyway.
The great thing about Stamp Duty is that it is a largely voluntarily tax. The bad thing is that it operates in bands, so that a difference in price of a few pounds can lead to a difference in tax running into thousands.