Confidence: It's likely to be this, but I can't be certain.
Notes: Surprising sometimes how far one can get with one photo! No doubt that it's Hypericum of some sort. One can see the translucent glands on the leaves, and even some black ones of the petals, which as Bill poits out are inrolled, typical Hypericum shape. Unfortunately one cannot see whether the stems are glabrous, but of the two hairy species one has reddish glands on the sepals (H. elodes) and the other lots of stalked black glands (H. hirsutum), both of which seem to be absent. One can then split the 18 or so remaining possibles into those with and those without black glands. This specimen has the odd black gland on the leaves and also on the petals, so not H. androsaemum (sorry Bill) which has none. But now we're in more difficult territory because we cannot see the stem (which can be quite diagnostic in Hypericum) but we can still make progress and narrow the field from the 10 remaining possibles. Several species (and hybrids) have distinctive sepals - H. x desetangsii has sepals with a denticualte apex and H. maculatum has obtuse sepals and so it's neither of those. H. humifusum has 2 sepals much longer and wider than the other three - again it's not that. H. linariifolium and H. montanum have more or less conspicuous black stalked glands on the edges of the sepals - so not those then. H. tetrapterum has petals about as long as the sepals, whereas in this specimen they are much longer - reject that then. Now it gets a bit trickier. Are there really black glands on the leaves? I think so - around the edges where you'd expect them to be if there were any. OK - H. pulcrum and H. nummularium have no black glands. And now we're down to just two possibles. H. undulatum and H. perforatum. Now we can only see the very young leaves underneath the flowers, but in H. undulatum they have wavy edges, as the name implies. So I'm going to stick my neck firmly out and suggest that this is probably H. perforatum. Which is probably the most common species. Good old Occam and his razor. Phew!