Dan mate as long as they've sent the e-cheque all should be well, they dont have any option to cancel.

All the ones ive ever been sent were fine but dont send anything til it clears.

I did actually have considerable problems with ebay just before xmas which caused me great hassle, and i ended up sending an item that had been paid for with an e-cheque, but it still cleared fine, i was worried when i realised the mistake but all was well