I have inadvertently put the wrong weekkiller on my lawn in patches and killed off the grass. What can I do now?

Whoops! Firstly water your lawn heavily every day for at least a week. The weather may help out a little. This will dilute any remaining active chemical. Then I'm afraid you are in for a wait as you will need to be sure where all the effected areas are. With a systematic weed killer this may take a few weeks. By this time we will hopefully be in summer so it may be difficult to keep the ares watered if reseeded or turfed. Ensure the effected areas are kept weed free whilst bear. Seeds are not effected by the chemical until germination so any seed, grass or weed may germinate before you proactively make repairs. Good Luck!