Turf lawn uneven, please help

[h0ffman]

I moved into a rented house about 2-3 years ago that had a turf lawn in the back garden. In all honesty, I haven't looked after it apart from the odd mowing during summer. Looking at it today, there are big patches where the turf has died, sort of corresponding to the areas which hardly ever see any sun. Also, the garden seems to have sunk in many places. Just to add some more info in, the garden actaully covers a gas tank and was a new build when we moved in.

I've been looking for what to do and it seems like I may need to re-turf it but thought I would ask on a good forum first.

Any help or pointer would be appreciated.

Thanks