Thanks Hotrod, Deluks and Noseall for the clarifications - I'll take back a few bags of sand (keep a few for later) and get some more ballast.
That point about aggregate being stone is NOT universally understood - I have absolutely definitely seen it used (even defined!) as synonymous with ballast - but I understand what I need now.
The mix-by-weight or mix-by-volume thing seems to be a quagmire too... the densities of ballast and cement are quite significantly different so mixing one way to a 5:1 mix would make a 6:1 mix measured the other way. Doing it by volume will give the highest proportion of cement, so I guess that'll be safe, if slightly overkill.
Yep, thanks. I understand now that those are the correct definitions, but I've seen aggregate wrongly defined elsewhere. If I come across the link I'll post it for you all to see and ridicule
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