I'm very fond of this tree/bush (pardon my ignorance), but I fear it's beginning to take over this corner of the garden. Any suggestions on how to trim it back without completely ruining it?
Your plant is Sambucus 'Black Lace' - its a rapid grower and if left, will reach 6 metres by 6 metres, like a small tree. Although it responds very well to hard pruning on a yearly basis, it is very difficult to keep it as a medium sized or small shrub, and the constant cutting that's required to do that would mean no, or very few, flowers.
The recommended time for hard pruning is actually winter, by mid-February but after November, so at this time of year, you should only reduce the longer lengths by half, and lightly trim the rest.