This is a red-and-black froghopper (Cercopis vulnerata) photographed in the park yesterday by Ian Bushell. There are ten different species of froghopper in the UK and while the red-and-black froghopper is not the most common, it is widespread.
Froghopper larvae are called nymphs and they are responsible for that thing called cuckoo spit. Cuckoo spit, which is not spit and has nothing to do with cuckoos, is a foam of plant sap which the froghopper nymph produces by blowing bubbles through its anus.
The nymph hides from predators in the foam which has an acrid taste to deter further investigation. Research has shown that the foam also provides temperature control for the developing nymph, mitigating extremes of heat and cold.
Here, just for fun, is a short video of a froghopper nymph blowing bubbles through its bottom.