Gazumping occurs when you have put in an offer to buy a house and at the last moment someone comes on with a higher offer. Legally the owner of the house has the right to change his mind and sell to the highest bidder. However it can be a frustrating business because the process of buying a house can be very slow, meaning that rising house prices encourage people to increase their offer at the last moment.
With many £1000s at stake it is worth considering various things to prevent gazumping.
See more on Gazumping in the UK Housing Market