Well, not 100% positive but think you are safe with 19mm.
Supaflex site says early cars are 18mm and later ones 19mm. My 1992 has 18mm; I measured before purchase with a vernier calliper and they were spot on 18mm; opened up caliper to 19mm and had 1mm slop.
If you've a good quality rule you could check the jaws of the spanner are accurate, alternatively try a 19mm spanner, buy a cheap 'Draper' vernier calliper ( very useful for all sorts of, er, measuring things )or wait a bit and someone here'll give you the troof. BTW when I replaced my rear bushes recently two of the bolts sheared......careful; took a couple of days to get the broken bolts out.