Got it sorted, but no thanks to Rapidshare.
Apparrently under their new guidelines you need to pay for downloads, which you do by buying 'Rapids'. These Rapids are used up with each download, depending on size, until you need to buy more.
I have a paid-up account until April 2011, but couldn't download until I worked out - on my own with no guidance from Rapidshare WHATSOEVER - that I have to convert the Rapids shown in my account to downloads. Once I did that, things were back to normal.
Very very poor information from Rapidshare - it appeared I actually had to buy more and thereby spend more money until I worked out that I already had a store of Rapids in my account for conversion, equal to the value of the six months remaining in my account.
Simple now I know, but very confusing until worked out.