Possibly a polygonum....persicaria amplexicaulis maybe ????

My fear is young shoots of Japanese Knotweed, a real nasty.

Definitely not Japanese knotweed

Thank heavens for that ;)
It's definitely a weed-type thingy though so I can yank it out?

It appears to be
Polygonum amplexicaulis according to
USA: NY:: Tompkins Co. Ithaca, Cornell University Campus at
http://www.gymnosperms.org/imgs/kcn2/r/Polygonaceae_Polygonum_amplexicaulis_6488.html
Whether it is a weed or not is up to you - I have it growing in my garden (came with the hse) and it has its uses eg for flower arrangements.
It spreads easily but is not too rampant, quite happy to have it in my garden...unlike some other plants eg bamboo...will return to that later...
This is my first response - I hope it is OK..
Lovely to see so nice things on your site...

My plant is here...ups - how do I do this...sorry have to pick the pic from my other -email address...will be back
- sorry cannot include a picture of my plant by copy paste method......

Brilliant, thank you. Mystery solved. It looks quite nice actually , I might leave it although it has taken over quite a bit of space so may have to cut it back a bit.

It definitely looks like Polygonum "Superbum". I had one at my old house, brought a clump with me to where I live now and will take a clump with me when I move into our new house! It has lovely toilet brush like pale pink flowers. It likes plenty of moisture and it is good for bog gardens. As Lm_bahr states, it does spread but is easy to control.