It could be a couple of things but if it is fast growing and woody, I would think it is Willow. Hopefully your remedial action will sort it but they are tenacious buggers. If all comes to all, you can use a weed wipe with Glyphosate but they are expensive.

Yep, willow. Goat willow, salix caprea, to be precise. Seed themselves everywhere, great colonisers! Dig it out.

Oh, didn't read the last bit. You cut it off and it'll grow back from latent buds at the base of the stem, so, if you can, dig it out. Far better than using any chemical which may just affect the blueberry. I say that because I have treated willow tree stumps with weedkiller in the past, and ensuing wet weather caused the weedkiller to affect nearby plants.

SD, luckily, no one heard. Quick, move on, next question!

If you want to use glyphosate without damaging anything else this is what I did : Make a tube of something, plastic, newspaper, anything will do, as long as the willow and wide enough for the leaves to be in their natural positions so the spray will wet them all and nothing else. Then put the nozzle of the spray down inside the tube and spray.May be a bit difficult if your sprayer doesn't want to work upside down.
Or you can use the new glyphosate gell which you paint on the leaved individually, but can't vouch for this as though I have a jar I haven't used it yet.

If you see you've got a few drops on the blueberry no harm will be done if you wash it off straight away with clean water (rain water if your water is hard)

SD I won't say a word to anyone.

So now I know what it is I'll keep a close eye on the pot an see if anything else comes to the surface. If anything does then I will follow Steragram's advice very closely, particularly as I've never used glyphosate before in any shape or form. Thanks Vincentdunne as well.
Strangely enough I had a few willows in a previous garden and kept them nicely trimmed but I didn't recognise the leaves. (Well it was over 20 years ago)
Thanks for all your help again. I think I need to do a diary type thing and record all the rights (and wrongs) and include pictures for reference.
I knew I had done the right thing by joining GOY.
Thanks everyone. xx

Re: glyphosate. There is a hand held wipe which is easy to use and safe in adult hands. It is, though, expensive.

Is that different from the fairly new one that you brush on? I've got some of that and haven't plucked up the courage to use it yet.

A hand held wipe seems to be the better way for me if anything rears its ugly head.
I'm surprised it was only one pot which was 'infected' as they're all close together. I shall keep a close eye on them all now. Thanks for the help.

That seems to be the one. Works well with dandelions anyway.