Cheers folks.
TomTom: I was almost right! Cheers.
I'm looking at tyres 215/40/17 87W for my Ibiza Cupra TDI (the standard tyres are 205/40/17 84W), so I'm guessing tyres with an 87 load rating will have thicker (and consequently) stiffer sidewalls than an 84 load rating tyre?
sash_girl_clio: nah, that's the "W" bit (W in this case is up to 168mph IIRC but it's the only rating available in the tyre in this size I'm looking at).