are the stems 'furry' or smooth. my first thought was Rowan or one of the Sorbus. If it is furry then it is probably Rhus typhina, or staghorn sumac.
Either way it will become a small tree so if you don't want it pull it out.

It looks like Rhus typhina to me too. In one property I had my neighbour had it in her front garden and it grew like a forest from all the suckers. She eventually had it removed.

I think that it may be a Meadowsweet (Filipendula ulmeria) as this has the same type of veined leaves and the reddish stem. My first thought was Rowan as I have just planted a self-seeded one up on the field, but the stem is not right for that. If you Google it and have a compare!
Meant to say that Meadowsweet likes damp places.

Meadow sweet stems have small 'stipule' leaflets along them and these don't appear to have them. The leaf seem to be too slim too.

Does your neighbour have a stagshorn sumach (Rhus)? It is quite capable of sending roots under the fence and trying to colonise your garden if that's what it is.

Thank you all. I will look up the names you have given and compare.

This is how the birds seed the berries of my Sorbus cashmiriana, in the gravel.