We had some mowing grass on a per bale rate about 8 years ago, was told £1 bale beforehand, yet after baling it changed to £2. We paid that year, but never dealt with the owner again, and neither has anyone else, been set-aside ever since.
On the weeds note, we rent a lot of ground on grass keep, and often spray weeds if they are a problem, but last year we cancelled our grassland spraying due to the dry spring and concerns of killing the grass as some of it looked stressed with lack of moisture anyway. Interestingly, I was chatting to a man from Dow at the dairy show and he said we did the right thing to cancel spraying - and he was the chap selling the stuff. Give your tenant another chance this year, and if he doesn't pull his socks up, chuck him out.
Handshake agreement is usually good enough for us too, but did have a written agreement once, the place was up for sale though, still have the grass keep from the new owner.