Posted: 20/02/2016 at 14:53
A mix of soil and compost spread over the area to fill the holes. Ideally you would pull a plank or something similar over it to get the level, but that might be a bridge too far. You can do it by eye instead - filling the holes and treading lightly to firm it. Do a bit at a time if it's a big area. You might have to do it a few times over the summer to get it looking more level, as weather will cause it to settle. Sow grass seed on those areas and a little extra over the rest of the lawn to blend it all in.
I expect you could get someone to do the job for you initially, if you felt that was beyond you, and you could just top up any little bits that sink at a later date.