Ok I've made a breakthrough in this(to me!)confusing 1.5" tapered steerer malarky and hopefully if anyone else is struggling this info will help. First issue was trying to source a lower bearing without having to buy a full Cannondale headset. Well I purchased KCNC complete tapered headset online for just over £30, it has FSA No42 compatibility and the bottom bearing is 45/45 and fits SuperSix/SystemSix headtubes and is compatible with the crown race that Cannondale supplies to fit its own 1.5 tapered forks. So one problem solved, a complete headset that works with Cannondales that is way cheaper and I'd assume there are many other compatible brands. Now the other issue is if you buy non Cannondale tapered forks(like I did) and find that the Cannondale crown race doesn't fit, well I bought a cheap set of tapered forks and made it fit with a lot of filing, I also bought a set of Easton SL tapered forks and found it no where near fitted, you'll find that rant on here somewhere! and I've also since come into contact with DEDA RS tapered forks that the crown race didn't fit either without heavy modification, well it turns out that Cannondale forks have a specific crown race fit whereas it seems every other brand uses a different but common fitting, the crown that came with the KCNC headset fits the Easton and DEDA forks, allowing these forks to be fitted to your SystemSix/SuperSix frames, I've also bought a separate crown race for use in a 1.5" FSA Orbit Extreme Pro headset and that works too. So hopefully now I've cleared this issue up for those struggling as much as myself, of course now I've sorted it Cannondale have now decided to go 1 1/4
Forgot to mention, if you buy an aftermarket headset like the KCNC I bought and run Cannondales tapered forks you will need Cannondales own crown race, the one that comes with the KCNC would be too big. Luckily the Cannondale crown race is an easily available part you can get from your Cannondale LBS.