I'm not sure what it is but maybe it's a fruiting plant which has been seeded by a bird dropping the seed.Unless you particularly like it, it may be best to remove because if it's grown that much in just this year then it's most likely to quickly become a giant and could cause irreparable damage to your wall.
It's certainly not one of the 'usual suspects' ie Elderberry, Privet etc

Rhododendron?

Did it produce any blossom or catkins in the Spring? It does look very much like goat (pussy) willow, I would certainly dig it out before it has a chance to damage wall or paving. If you like it try growing it in a container so that you can control its growth more easily.

Thanks alot for the comments. It didnt produce any flowers or catkins in Spring. Would an alternative to removal be radical pruning so that it puts its energy into top growth rather than roots?

No, in short. In order to support topgrowth, the plant naturally extends its roots. Have it removed, or chop it down and poison the stumps, asap before it knocks the wall down and lifts the pavement.

I see what you mean - willow type leaves and eucalyptus type trunk - but whatever it is I agree with the others, it needs to come out.