I've had that before. I've been told they're actually not allowed to leave a parcel in an insecure or exposed location. I think they're allowed to leave it in a porch or another sheltered location, providing it's hidden. In my experience, Royal Mail are pretty good about it if you complain to the delivery office (not the Post Office - they have nothing to do with deliveries). It's usually a temp that's responsible.

OTOH, if it was supersaver, then you're probably dealing with a local part-time courier working directly for Amazon.

In any case, if it doesn't arrive, then legally it's the seller's responsibility to make sure it's replaced or refunded. It never ceases to amaze me how many eBay sellers don't get this!