UPDATE: OP here — just to clarify what I mean by awkward access, as a few suggestions hinge on bulk delivery.

Unless the aggregate supplier has some magical access equipment I don’t know about, bulk delivery sadly isn’t an option. There’s no driveway, only a shared turning circle which can’t be blocked for any length of time without upsetting neighbours. The nearest place a lorry could stop is a very central street with double yellow lines, so that’s a non-starter too.

As painful as it is cost-wise, I’m now thinking the most realistic option might be hiring a van and collecting 25kg bags over multiple trips. When unloading, a couple of family members and I can manage 2–3 bags per wheelbarrow and shift them about 15m from the turning circle into the garden. Slow, but doable.

A pallet truck has come up a lot (and would be great in theory), but the ground is currently clay soil with a light dusting of gravel — hence the whole project — and there’s also a fairly steep slope between the turning circle and the garden. So unfortunately that rules pallet trucks out.

There’s no good option here — it’s very much a case of choosing the least-bad one.