Can anybody advise? I've mixed Addition Cure Silicone Rubber and catalyst by volume and not by weight. Might it still cure? 

If you look at the Technical data sheet you will see that the density of the rubber is 1.08g / cm3 and the catalyst is 1.07g / cm3.
I don't see any problem as there is less than 1% difference. Most scales only measure whole Grammes so you can expect to be at least 1% out on small mixes and they still set.

Brian.

Thank you very much x

you should be ok on such a small error due to the similarity in density.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant