Most tuning machines have exposed gears, which your Warwick Corvette does not. If you like the concealed gears, then you should look at Gotoh. If exposed gears are acceptable, include Hipshot and Schaller:
http://www.allparts.com/Bass-Tuning-Keys-s/171.htm http://www.warmoth.com/Tuning-Machines-C208.aspx
Some are sold individually, others in sets, hence the price disparities. The per/tuner cost ranges from ~$13 to $45. If you go for the cheapest, don't expect wonders.
You will need to know the diameter of the holes in your headstock, which will probably require removing one of the tuners, and measuring accurately. Even if you don't need to enlarge the large holes, you may have to drill new (blind) pilot holes for the anti-rotation screws.
Most of the manufacturers of tuning machines maintain a database that cross-references basses to their appropriate replacement tuners. So far I have only queried Hipshot, regarding Ultralites for a Squier, and they responded promptly to my e-mail.