Four years back, I switched to a grain free food. My cats trimmed down nicely with no work on my part.

Also, don't leave food out all the time. Set food out twice a day for half an hour, morning and evening. End of the half hour pick it up put it out of reach.

If you can also work in a play time a few times a day, with a toy on a string, that will help. Make sure to put the toy away at the end; the toy on a string is dangerous if left around.

I don't know which brands are available to you (I am in US), but I use this site to find a good high quality grain free food:
http://www.petfoodratings.net/cattable.html