Hawthorn Shield-bug (Acanthosoma haemorrhoidale)
This insect is shield shaped and is found mainly on the leaves and fruit of Hawthorn so it has the common English name of ... Hawthorn Shield-bug!
Shield bugs are part of the order Hemiptera, sub-order Heteroptera; they are not flies or beetles, they are a separate taxonomical group. They can vary in colour quite considerably depending on age but the red triangle is usually visible on the Hawthorn Shield Bug.
These are insects that tend to hibernate as adults and so are most frequently seen in spring when they emerge or in autumn as they are looking for somewhere safe to pass the winter. That said, they are not uncommon in summer either and are frequently seen in gardens.
One of the bigger shield bugs, one of the most distinctive in appearance and and one of the more frequently seen, there is likely to be one near you soon!