Once the hammer drops you are committed
to buying, and buying quickly - about 7 days you have to pay up as I
recall.
10% deposit on the day paid by cheque and the balance 28 days later is, I
believe, more normal today (the last 2 I went to did things this way, but it
varies between auctioneers). You do not need to have the finance in place
before the auction, beyond the deposit, but you have to be confident that it
will be there within 28 days so talk to a few building societies in advance.
At one auction there was a mortgage adviser on hand to sell their products
on the day (it's better to shop around of course). If you fail to pay up,
the deposit will be lost and you will be liable for any losses the seller
suffers.
Most auctioneers will be wary about accepting cash.