It's for the PY cargo meeting. As long as it fits these criteria, it really doesn't matter what it is.
Criteria:
1) About 80kg of it
2) Not hugely bulky.
3) Easy to transfer between bike and trailers
4) Unlikely to damage or soil a bike/trailer
5) Cheaper than £3.62.
£3.62 is the price of two 40kg-ish bags of concrete ballast from the Wickes down the road. Bizarrely it appears to be more expensive to buy loose sandbags that I could fill with free dirt than to pay for the aggregate itself.
The ideal thing would be weight training plates or dumbells, and infuriatingly I kept 80kg of them in my flat for years, but when my knees busted I gave them away. The aggregate bags look to be a close second.
If anyone happens to be planning DIY or gardening that need aggregates or compost, then I'll get it and sell it on to you.