Lots and lots of humus rich material like composted animal manures, spent mushroom compost, leaf mould, things like that. You might find it cheaper to locate a major supplier and have it delivered in bulk, depends what suppliers are around you. The only problem with a bulk delivery is shifting after its been delivered... horticultural grit would be useful too. If you have any mushroom growers in the area, its worth contacting them, equally stables, but make sure, if they have manure, its been composted for at least a year, preferably two, with, for preference, straw.

Its worth making the investment now, its the one time you'll get a chance to improve the soil, and good soil grows good plants.

Thanks. That's what I thought might be said. I agree, it's the one chance I'll have to improve the soil, and I'll kick myself if I don't.

Will look into those options.

Lateral thinking - a hollow that size could be made into a sunken garden or a rockery with room at the base for some chairs and a little table? (Memories of of my childhood garden where Dad had done this in the hole the Anderson shelter came out of. It was a lovely sheltered place to sit and added lots of interest to an otherwise flat and ordinary garden) It would greatly reduce the amount of topsoil and/or humus you would need, not to mention the digging.