A diamond blade will whizz through slate no problem, however it will leave a clean cut edge, not what you want if it's part of the slate your going to see! What you need is a slate axe an old bit of steel or a block or basically something with an edge to cut along. Then draw your lineaway! the slate where you want to cut itand axe away! Don't try and take to much at once, you'll geequipment I'll e hang of it!
I have taken a picture of the equipment I'll try and post it now but I'm on my mobile if not ill post it later
Also forgot to say mark your line on the back of the slate then cut it from the backwith the axe and it leaves the edge with the riven effect you see in the front. Also slate axe are left or right handed so make sure you get the right one for you if you get one!
yea the edge of a block will work fine you'll just find after a while it wears away a bit but just start using another edge then!
the trick is to hold the slate at abit of an angle to the edge of the steel/block then axe away! theres a silverline one on amazon for £9! www.amazon.co.uk/Silverline-868715-Slaters-Axe/dp/B000ND8SZI its the same one i have and have done many a roof with it! and yes as dannos video shows its really not hard!! some slates are tougher than others to cut but you'll soon get the hang of it!