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I speak to my neighbours but as an Australian, my level of mad is certainly non-zero.
If you're going to do the work anyway, you could suggest "the first one's free, but in future, if you could keep this stuff under control, it'd be better for both of us"
Really, before you started the work you should've asked them to prune their stuff. Because if you do it and then just dump it on them, you look like a bit of a prick even if that's not actually the case.
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What hippy said. Also, if it's a bit of a trek to find the house responsible, it's pretty likely that they are equally as unaware of what is going on behind your garages as they probably don't go down there.
Have a chat first, see how responsive they are, ideally agree to some ongoing maintenance from their side.
In London? Are you mad?
Also, what do I say - "Your laziness has caused me five days of graft and a few hundred quid of costs, please keep your plants under control in future?"