The gearden has been left in a mess after building work on the house. What is the first step in preparing the garden to relay a lawn. Is it applying weedkiller and then rotavating. Any advice welcome

If you put weedkiller on the weeds, which I assume you have in the 'mess', then only use a systemic one, not one that will stay in the soil, or your lawn will die too! Once the weeds have died, and you have removed all rubble etc. I suppose a rotavator would cut down the digging to get the roots out. You'll probably have to dig in some compost. Then you have to remove all stones from the surface, rake it to a fine tilth and then either lay turf or sow weeds. Turf is obviously more expensive, but quicker!

Good luck.

I guess you still have a bit of what the developers say is a 'lawn'. In which case strip the turf off leaving about 2 inches of sod beneath the old grass. Relay it upside down then import about a foot of new soil on top. Then lay new, quality turf or seed. Lots of work for sure, but ultimately it's quickest and cheapest and you won't have to keep watering a yellow lawn in the summer.