Wenderlynne, I think I remember you're in England; is your friend there too?
The soil doesn't freeze, right?
When/what are they hoping to plant?
What sort of area are we talking?
In my climate the soil never gets
really cold, so my ideas aren't from
experience...
nustada asks about your comfort levels with natural/unnatural materials.
Black polythene is the standard soil-warmer in NZ, but it's
far from natural!
I wonder about hot-composting in place? It certainly generates a massive amount of internal heat, and I assume that would perculate down to an extent.
They could do windrows, following the garden shapes.
A really basic heat-producing mix like
lawn clippings (
local lawn guys will drop as much as I'll take) and shredded office paper (there's literally
tonnes of it, but not everyone's comfortable using it)
It would get very hot fast, and with a few turns, break down fast too. Clippings get so hot, I don't think you'd need the standard cubic meter of material.
It'd make boring, not very nutricious compost, but it would serve its heating purpose...
I'd also loosten the soil with a fork before building the windrows.