Buying House - Stamp Duty and the approaching Budget

We are nearly in a position to sign and exchange contracts to buy a house.

Hypothetical question...

If say, the budget does away with stamp duty as of midnight on the budget day (or change the rates of stamp duty)...

What date decides whether we would have to pay the stamp duty or not ?

Is it the

- date the contracts were signed
- the day we get the keys
- the day the money exchanges hands
- the day of closure ?

Any ideas ?

(This could save us over 15k in stamp duty if we delay and hypothetical becomes reality - but then dont want to jeopodise the purchase)

If I recall correctly, previous Stamp Duty changes have taken the date that contracts were exchanged as being the relevant date.

What has happened in previous budgets where a contract is signed before and closes after the budget is that you seem to be allowed to cherrypick the favourable date.

This may or may not happen this time.