The answer for soils which are too light, sandy and free draining is the same as for heavy, clay soils - humus rich materials, so you want things like garden compost, composted animal manures (bags at the garden centre), in other words, what you're describing as 'soil improvers'.

Topsoil is sold in relatively small bags at the garden centre, but isn't usually described as 'multi purpose', that term is usually reserved for potting compost. Works out expensive to buy topsoil this way, but ordering a ton off the internet or wherever can be a dodgy business - it often turns out to be motorway spoil, full of weeds and stones. If the soil your landscapers are talking about removing isn't just skimming off the lawn, but taking off a good depth of topsoil, ask them to conserve and replace it where its needed, particularly if they are going to remove more than a spade's depth and you want to plant afterwards.

Get in touch with a reputable dealer in your area that can deliver top soil to you. It's far cheaper buying in bulk, either loose by the lorry load or in those huge builders sacks.

I forgot to say ask for 'screened' topsoil if you're going to order some in, that should at least exclude the larger lumps of rock.

Thanks both. The multipurpose top soil is one I came across on a website selling in bulk this morning.

Reusing skimmed top soil may not be the best idea. The garden is riddled with mares tail so I'm expecting the soil to be full of roots. Fingers crossed getting rid of the top layer will make it more manageable.

From your comments I'm now contemplating using a mixture of compost (of some sort) and screened topsoil. What do you think?

Mare's tail, oh dear, frankly, skimming some off the top won't solve that problem, the roots can go down 2 metres once its established, so removing the topsoil for a couple of inches might get rid of the rhizomes nearer the surface, but won't touch the deeper, more established roots. In which case, its still worth asking them to keep your topsoil for reuse... you'd see the rhizomes quite easily in it anyway.
You might wonder why I keep saying keep your own soil - its years of having topsoil delivered from various places, screened, unscreened, , all of it full of weed seeds and some of it full of pebbles, and most of it clay once it was wet. Apart from one lot, which was described as 'loam' - cost an arm and a leg in comparison, but it was good.

Yep pain the the whatnot. One of my first jobs is going to be digging the whole garden and removing as many roots as possible. I know I'll miss loads, I'm just hoping to reduce the problem.

Importing more weeds definately isn't part of the plan. I'll speak to the landscaper on Monday morning. Ill probably just get some compost to improve the soil a bit. Thanks for your help.