I had tried both angle grinder with carborundum disks (wore em out in seconds) and chipping with a cold chisel (worked, but not on long thin bits: They always cracked across) and so decided to hire an angle grinder with diamond blade.
What a difference!
I hired it for a day, but did the job in just over an hour, cuttng all te awkward bits around edges and steps in that time.
Like a knife through butter.
Cost me £17.92 and saved a whole load of broken slabs. I even cut down some other broken ones to make new smaller ones.
Only problem is the cut is too clean. Still a couple of minutes of distressing with teh cold chisle solved that :-)
Just passing this on to anyine who is doing similar.