This thread is very helpful. Ive decided to go for a magnetic ballast rather than a ventronic digital which is £50 more expensive than the magnetic.

This is also what they say about digital ballasts tho:
"Q: Will this ballast cut down my electricity bill? I use a budget 600w lighting system at the moment but electric bills have gone up almost 3 times the amount that it was before.
In a word, no. Unfortunately there is no such thing as a ballast that will cut down on your electricity bill other than a lower wattage ballast (250W & 400W). Other retailers claim that their digitals ballast use less electricity than standard ballasts but it's complete and utter rubbish, they are simply trying to sell you a product that want, rather than one that will do as you need. If you see electronic ballasts being advertised as using less electricity then steer well clear of the retailer, they are either very poorly informed or a typical "hydroponic" rip off retailer. 600 watts of electricity is 600 watts of electricity which ever way you look at it, it's basic physics and as far as we know can't be altered.
Digital ballasts do have some advantages i.e. lower heat output and in the case of the Ventronic it maintains lumen output at the maximum level as the lamp naturally starts to degrade after a couple of months. The Ventronic is actually a 650W ballast, it produces extra around 17% extra lumens when used with Sunmaster 95,000 lumen lamps but will in fact probably use slightly more electricity than a standard 600W ballast to achieve this."
Grow Lights > Ballasts > Ventronic 600W Digital Ballast