As its not till next year, the simplest thing to do is regular cutting of the rough grass. Cut it every fortnight, weekly from May onwards, up until late autumn, and cut it in winter if the weather is suitable.

Otherwise, skim off the rough grass and weeds, dig or rotovate (provided there are no pernicious weeds such as docks or bindweed, when small root pieces will just be chopped up and will grow again) level and prepare for turfing.

Thank you Bamboo. My husband put the heavy duty strimmer over it this afternoon and it looks very rough and bumpy. It is an out of the way bit of the garden that used to have a cover of wild cherry trees until they were broken to destruction by snow 3 or 4 winters ago (we are in the north of Scotland) and taken out for firewood. There are still a few suckers from the old trees but no difficult weeds like you mention. I did wonder about rotovating but am not sure how powerful a machine I would need to hire.

Welcome to GoY Jeanbbell, Bamboo's advice is good. Could you edit your 'About Me' so we know which part of the country you are in, there are lots of us from Scotland - Bulba and I are in Moray...

talk to a marquee company first and get them to view the site. The one the neighbours used had boards that they used to 'level up' the ground.
So the work in the garden may not be as onerous as at first thought.

Thank you everyone, some really helpful ideas there. I have edited 'about me'.

I would buy a ton of top soil and sand. Spread it level over the whole area of lawn and then buy turf and lay that on top. You then have what you want - a nice level lawn.

The other way is to add top soil into dips to level them, then seed the top soil. Cut into the turf with a saw and remove underlying soil to reduce bumps. It should look fine by next summer. This is the cheap option.

When I said just cut, I need to clarify that I mean with a mower, specifically, not just a strimmer, if that's possible. I saw my father in law start mowing a bumpy, rough grass and weedy field and a year later, it was sufficiently smooth to use for golf practice. Although I admit, he did have a sit and ride mower, which made it a lot easier.

Thank you everyone. After some deliberation we are going to persist with mowing and be ready to fill in with topsoil if it is still uneven in the autumn.