that could be a wasp nest or more likely a begnin growth. the plant will have been damaged at some point and it has produced a callous. check for wasps and if you want too you could remove it. personally I'd leave it alone.

I think it is probably a nest as well, see if you can see some site of entry, wasps don't usually use the same nest each year but make sure it's empty before you move it, Derek.

That looks to me like a proliferation of growth in reaction to damage, perhaps a wire or rope which has cut into the tree stem. Check the lump for woodiness. A wasp nest is papery. If it is as I suspect, I've seen exactly that response in trees I've been asked to treat. Interesting. If you cut it out it'll take a while for the hedge to make up the gap, but leaving it in there won't harm for a while yet, unless it increases in size and the stem bends under the weight. What tends to happen is that the stem bearing the load thickens in response to the weight and you end up with a huge burr (woodcarvers love 'em)

Thank you for the comments. It is not a wasp nest although I have had one of those in the past which I discovered rather painfully when I was trimming the hedge.

The lump is getting bigger year by year and I think I will probably cut it out now. Thanks for comment about woodcarvers.