No, it wont kill it, its a hardy old beast. I usually found it was necessary to cut it back a fair bit around this time of year because its such a rapid grower and tries to take over, so take off whatever's in the way or out of control, though I'm not sure I'd hard prune the way one does in winter.... just enough so it still looks vaguely attractive, and take some more when the leaves change colour and start falling in autumn.

I’ve done what Bamboo has suggested. I cut off any branches that are spreading too quickly or getting in the way. Like she ways, it’s a hardy beast. It’s part of the elder family so grows quickly. I love it.

Isn't it true that f you prune now there is still time for it to grow some flowering shoots for next year but a hard prune in winter will result in no flowers? -
But it doesn't matter if you aren't worried about it flowering.

I think that's right Stera. I quite often chop bits off mine at this time because they can get really heavy. As Bamboo says....:)

I prune my sambucus nigra hard every spring, but it still carries a few flowers

Well I do love the flowers but I'm not bothered about the berries. However, this year I need to take drastic action so I might have to sacrifice flowers for next year.
Thanks all, for your help.