Thanks gents, after looking at multitude of articles and forums it sounds like the overwhelming consensus is as Elwood says, use H4831sc rather than Viht.

I wanted to go for a Viht powder to keep cost down and for ease of availability, but if most people use H4831sc then there must be a good reason for it.

Anyhow, just took the plunge and ordered a few pots of H4831sc. Price of the stuff is eye watering compared to Viht powders and nearly twice what I paid for my Elcho 17. I'm too tight for this game really, archery has to be the way forward...