Under English law, the price you agree on is not definite until the `exchange of contracts'. Up until that time (usually 6 - 8 weeks after the seller accepts your offer for the property), the seller can choose to cancel the sale and accept a higher price from another buyer (if the market is moving upwards). Likewise, you have the right to cancel the agreement and buy a cheaper property (if you feel the price you originally offered was too high, or that prices may be falling.)
When the seller takes a higher price (or asks you for an extra £10,000 at the last minute), this is called `gazumping'. When the buyer demands a price drop at the last minute (usually to compensate for decling house prices in the area), this is called `gazundering'.
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