Anyone know what this is?

[stealthsquirrelnutkin]

It appeared in my garden last year. I tried to dig it out but am a bit feeble, and some of it was hiding behind that segment of metal fencing that has a clematis twining through it. Moving the fence segment would have snapped the clematis so I gave up and forgot about it.
This year I decided to let it grow to find out what it is, but it seems to be flowering now (if those pathetic little white things at the top of one stalk are flowers) and I don't want it to self seed.
Should I kill it with fire?

That last picture is a bit fuzzy, hope this is a bit clearer.

Hemp agrimony

Thanks, that was quick. So it hasn't finished growing, or started flowering, but the good news is that pollinators love it, and it would cost 12 quid to buy from the RHS, so it has been granted a stay of execution.

Chuffed that it is also called ague weed, one never knows when one might be struck down by a bout of the dreaded ague.

The Swedes say that "beloved children have many names" which indicates this must have been a very popular plant in days of yore :
Eupatorium cannabinum aka
ague weed
andurion
bastard agrimony
common hemp agrimony
Dutch agrimony
hemlock parsley
hemp weed
holy rope
Indian ague root
water agrimony
water hemp

I'd cut the flower/seed heads off, and let it continue doing a good job of disguising the wall behind.

Yes, the wall takes a lot of hiding. I had managed to get it covered with native ivy after about 10 years, and then an odd job man that I hired to take down a rotten pergola did me the "favour" of pulling it all off and taking it to the tip along with the bits of pergola. So much for all the bees and birds that were enjoying it.

Hah! I’m so glad you gone from hating it and wanting to burn it in the fires of hell to granting it a stay of execution because ….. £12.00 from RHS. 😂😂

What can I say, I'm a cheapskate.

You could maybe propagate from cuttings and sell them on 😄 or let it seed and sell the new plants

I'm impressed by the people who are organised enough to propagate their plants and sell them on ebay. I could do the propagation, but not the selling, and especially not the packing up and posting.

It spreads a lot by the seeds from the flower heads so might be worth removing them once they’ve died.

Some information on its ague related properties here: https://botanical.com/~botanid5/botanical/mgmh/a/agrim016.html