Where do you live?

What kind of grass do you have?

Generally fertilizing with chemicals in the summer is not advised. Wait until the summer heat breaks. For many that time is early September. But if you are already cold by Sept, then maybe August works better. Again, need to know where you live.

You could use an organic fertilizer in the summer.

Would you be doing a complete renovation or just thickening up the lawn? How big are the bare patches?

Hi, I live in London, UK. No idea what the grass is, I'm afraid ;-)

It's not a massive lawn and yes I want to thicken it up mainly to help the bare patches which are maybe 18 inches square

"It's not a massive lawn"

I guess being a Lord ain't what it used to be, eh? Don't you have "people" to take care of this sort of thing? ;)

Many thanks for the help, guys! Ted: you're right but all my family's people went to the New World when our empire crumbled and now I have to tend to my own garden ;-)

That's cool, tending to your own garden isn't that hard. We all do it with marginal success!

There is never a bad time to use an organic fertilizer on your lawn. You can either use a commercially bagged organic fertilizer or you can use the raw ingredients that go into bagged fertilizers. Those ingredients include ground up soybean, corn, alfalfa, wheat, flax, and other grain type materials. You should be able to buy ground corn or alfalfa pellets (rabbit chow) at your local feed store. You might be driving past them all the time and not noticing them, so here is a link to find them in your neighborhood. If you know anyone with horses or even chickens, they will know where all the feed stores are. Call around to see what grains the stores have in stock. They should come in plain 20-kilo bags. The application rate is 6 kilos per 100 square meters. It is hard to overdo the amount. If you bury the grass, then that is too much. Organic fertilizers work completely differently from chemical fertilizers. What that means in practice is you don't have to worry about burning the grass.

That's great - many thanks for the comprehensive advice - appreciate it