I can do most fitting/fettling but I find it really hard to get this right. Anyone have any top tips or secrets to getting it right.
I did find getting an SLX chainset on one bike made it much easier so is changing chainrings the answer?
At the mo on my other bike I can get it to either shift up ok, or down ok, but not both together - I've faffed with the height, angle, limit screws and cable tension (and these are new cables). On the rare occasions it seems ok for a bit the next time I get on the bike it won't change properly again - even though I've tightened everything up.
Any pearls of wisdom?
Get it so that it just clears the big ring when fully extended (about a 2 - 3mm gap) and make sure it's straight. If you do that, the rest is easy
If you're finding it that hard, take it to a bike shop.
Try in this order.
Set height 2-3 mm above big ring
Set angle, outer cage parallel to big ring.
Set limit stops
Set cable tension, probably the most time consuming to get right.
Small adjustments are best, just take your time.
I stunned myself last time by having it mounted, set up and working in about 10mins last time i did it on my mates Marin! 
as bigyinn says (must be in that order), but another tip is set the lower limit screw too tight when putting the cable on, then when the cable is on, back it off. That way there is still that little bit of tension in the cable on the small ring position.
Just Infinite Monkey it, if you fit it an infinite number of times sooner or later it'll be perfect.