Previous knotweed, need to level garden for lawn

Shortly after moving house we discovered we had some Japanese knotweed in our fairly neglected garden. Fortunately it only seemed to affect one border and after doing some research we treated it with high strength weedkiller (glysophosphate) over two summers which, touch wood seems to have done the trick, but we have only had one clear year. I desperately want to seed a lawn but the soil is fairly uneven, with areas of higher soil on the border where the knotweed used to be. It is also stoney compacted clay soil with poor drainage. Everything I read about seeding a lawn indicates we would need to dig or rotovate the soil and level it before applying manure or topsoil, and I also want to do this to improve drainage. However I don't know what to do in the area the knotweed used to be, I don't want to contaminate any equipment or spread the soil from that are area to the opposite side of the garden where it is lower and needs building up and risk spreading  what could just be a dormant problem waiting re-spraying. But if