Both, in that order. Lawn Builder builds the base up to finished height, so the lawn sits level with your paths and edges. Topdressing Mix goes on top as the final layer. It's finer, so you can rake it smooth and sow straight into it. On an existing lawn you only need Topdressing Mix, to fill hollows and help new seed take.

Get on top of them in early spring, before the weather warms up and they flower and seed everywhere. Dig the bigger ones out by the root. A thick, well-fed lawn is your best defence, because weeds struggle to take hold where the grass is dense. Feeding and oversowing thin patches does more in the long run than spraying alone.

That's usually thatch: a layer of dead grass and roots that builds up between the soil and the green leaves. It stops water and fertiliser getting through. Dethatch in spring or autumn with a scarifier, which you can hire from the yard, then top dress and oversow the thin patches. Our dethatching guide walks you through it.