I know that feeling all too well, Nfk - a couple couldn't wait to buy my house, apparently, and wanted exchange and completion asap.
Eight weeks down the line when I'd found somewhere I wanted to buy and was all set to sign the contract, their 'cash offer' turned out to have been an anticipated sale which had fallen through, so everyone lost out.
I understand that Agents can only go by what prospective buyers/sellers tell them, but sometimes I feel more checks could be made. The system in Scotland seems fairer, but then again if the money isn't forthcoming at the end of the day what happens then?