More pictures, thanks for help, think you may be correct.

It sort of looks like a mint bush, but then I found some over 5 foot high.

its not Japanese knot weed is it ? its illegal to grow it here its so invasive . the stalks are quit brittle and have parts in them a bit like bamboo canes . its really a nice looking plant and has very big leaves . if so get some membrane over it to kill it and look in the neighbours because if they have it you will get it back .

Can you add a photo of the whole plant growing as well as this good one of the leaves? that will confirm whether its the dreaded knotweed or not.

its not knot weed, wrong shaped leaf.
a photo of the whole plant and if its in sun/shade etc. do your neighbours have it growing? do you see it in the roadside verges etc. [if yes to the last one it is probably a wild flower/weed.]?

Hi, I don't have any neighbours, we are in the middle of farmland so only potatoes and wheat fields nearby, all well kept. When I broke the stem it was clean white wood inside. Thanks.

the first plant picture does resemble it but the second plant is completely different ? remember a weed is a plant in the wrong place .

is it hollow and really quit brittle ie the top picture ?

Yes, a bit hollow. The second picture is in the veg patch, though it might be a herb, the flowers are just coming out, might be better able to identify it then.I did think they were the same family, because the shape of the plant is the same, but can see now the difference side by side.

The one near the bamboo is very large and I am afraid to pull it out in case it is a nice flower or something later. Also will leave a big hole I need to fill. However, I really don't want to encourage anything nasty that will effect the garden.

I have only been here two months and the garden is a little overrun (a lot in some places), but has obviously been well stocked and looked after previously. Identifying the plants is the hardest thing to do.

Thanks for your help.

I'll confirm none of the plants pictured is Japanese Knotweed - don't know what they are though!

Thank you. I will continue to make further investigations.
Will also be posting some other mystery plants up later.

The top plant I think I used to have but which I got rid of because it spread quite rapidly and threw up lots of plantlets. It had dark pink Achillea type flower heads and grew quite tall.

I wonder whether it could be Clerodendron bungei, which gets around 6 feet and seeds about a lot.
My other thought was Eupatorium 'Chocolate' but I think maybe the leaf veining is wrong for that.
The middle photo I have no idea.

Thanks very much for this reply, I think possibly Clerodendron bungei also after looking at other images on internet. I will save them, but maybe curtail their spread a bit and see what happens when they flower. Thanks again, I very nearly dug them all up!

Yes that's the one. I've found a picture of it in flower.