I would go for a 1:5 cement/ballast mix. What philjjj is talking about is cement/sand mortar mix.
If the ground is dodgy at all include a 6mm mesh reinforcement 50mm from the bottom.
Have a look at the Party Wall Act which is available to download free or have a chat with your friendly neighbourhood Building Control Officer at the council
I assume your footings are at least 150mm thick.
It would be more usual to set your mesh 50mm from the bottom, not the top. I would do this with small pieces of concrete brick or purpose-made spacers available from building supplies.
If you're using natural slate or imitation slate (not interlocking) the principal is the same in that you always have a minimum of 2 layers with 3 layers at overlaps. The spacing of the battens (gauge) is calculated by deducting the overlap (usually 75 to 100mm) from the length of the slate then div...
