Posted: 30/06/2015 at 18:32
After seeing your lawn I would have to agree with Verdun although you might get away with a resurface rather than a total dig up and level.
A deep scarification and a good mix of compost/topsoil with seed might provide the quickest answer. You will still have to fill any holes and level it but I think it can be salvaged with no problems.
Now is the time to prepare for this so you can sow at the end of summer.
You might like to try a small area in the worst part and see how it responds.