From the RSPB website.
Buzzards principally eat small rodents, but also take birds, reptiles, amphibians, larger insects and earthworms. Prey up to 500g is taken by active predation; anything heavier is usually carrion or seriously enfeebled individuals.
Gamebirds are sometimes taken, though these make up only a tiny proportion of the diet. Buzzards are more likely to feed on carrion.
The feeding grounds are often shared with other birds, though hunting perches are defended. Buzzards use three main hunting techniques. They locate prey from a perch and then fly directly to it. They may also soar over open terrain, occasionally hanging in the wind before dropping on to the prey and following up the attack on the ground. Alternatively they may be seen walking or standing on the ground looking for invertebrates.