I saw it every day, these last weeks, well hidden below a maple leaf. It is Phaneroptera falcata, the sickle-bearing bush-cricket, a species of bush-cricket. Its elytra are of the same color and pattern than the leaf so you, and predators, can hardly see it with the exception of its red eyes.
Due to its small muscles, it does not jump a lot but can fly.
Females lay down eggs inside leaves like those of Prunus spinosa (blackthorn, or sloe) which are very common here.
I found a juvenile of the same species in spring.
Phaneroptera falcata has been extending the northern limits of its range in mainland Europe in recent decades. Vagrant adults are occasionally found in Britain, and a small, but apparently established, colony was discovered near Dungeness in Kent in 2015.