If you've been plagued by mosquitos in this hot weather, it's because they are breeding somewhere nearby, in stagnant water.
I realised they were breeding in my rainwater gullies, which look like the picture. I poured a bucket of water down each one, which seems to have flushed any larvae away and makes sure that the trap (like a U-bend) has enough water in it to seal.
It works. I haven't been bothered since. I had already checked there was no stagnant water in my garden, and turned a couple of old water butts upside down; mosquitos only need a very tiny amount of stagnant water to breed. You might have a different design, or there's some other sort of gully arrangement, perhaps on a hardstanding for water runoff. Another bucket of water down each one after a week might be a good idea, too.
Some will tell you to drop a little oil into the water as it stops the larvae from breathing, but as water from stormwater drains isn't treated, this is frowned upon; the oil ends up in the rivers or the sea.