Potting Mix for cuttings

I want to ramp up my dahlia growing.
I'm having a kitchen extension and a patio area - woohoo!!
The patio area has been dug and it looks like I will have the opportunity of either adding a couple of beds or at least making one that I have much bigger

Anyway, back to the cutting mix -
When I've taken dahlia cuttings before - it's a bit hit and miss. I don't get consistent results. The stem can rot off - my mix is too heavy.

Many people use half and half -peat and sharp sand mix. 
I've just used MPC - sometimes with some grit. I've always just done the cuttings in mpc and been ok with the successes but I really want to get this right - I've only got 1 of each variety of some tubers so I don't want to lose those. 
Any recommendations of what to buy? I don't really want peat - I don't want to spend a fortune either. Can I use builders sand as opposed to sand from the GC?

Sharp sand is better imv as it is coarser than builders sand which can be claggy. I'd stick with the mpc and grit personally, maybe try more grit in the mix. The tubers should produce plenty of material for cuttings though. Are you just laying the tubers in a layer of compost as opposed to burying them, that way you see the cuttings easier and produce more cutting material. How are you covering the cuttings, in a propagator or plastic bag, maybe they're getting too humid and you need to let a bit of fresh air in each day.