Well if you really aren't sure then don't buy it, I find it very hard to believe you can't get an NTFS drive to work under Linux that was working under windows, not you personally but I mean in general, if it works in Windows it should work in Linux its just a case of finding out how to make it work. I would start by formatting it as NTFS in Linux then using it in Windows and not the other way around, I have never had a problem this way round?
In regards to returning the product I think they are lawfully obliged under the Distance selling Regulations? I have sent stuff back to OverClockers before within the first 7 days of purchase and they simply test the product and to make sure its OK and then refund me the full amount (obviously I had to pay to send the product pack though).
Also you could try Scan Computers if you haven't already?
http://www.scan.co.uk/Shop/Computer-Hardware/All/Hard-Drives-Ext/750GB-2000G...
HTH