Just measure each wiperblade and buy the replacement according to those lengths (I say each blade because on some cars one is longer than the other). I've not found that cheaper ones are actually good value for money as they seem to need replacing more often ! (I get mine - Champion - on line for about half the price charged in Halfords !)
Cheers, they are only cheap because they are half price. (usually �8, but now �4). Not too sure if that is cheap or not, but sounds good to me, even if i only get a year out of them :)