It is horsetail, a species of Equisetum.
It is a primitive plant, and it is unusual in some respects. For instance, one of its common names is 'scouring rush', which comes from the fact that the stems contain abrasive silicates, making them useful for scouring cooking pots and utensils (handy to know when camping).
It takes a bit of work to ID the species; perhaps you can do this for yourself if you're interested; guidance follows...
Here's a list of the British species, with short descriptions
You can also refer to the BRC's distribution maps to shorten your shortlist of candidates.
Based on your photos and brief description, I consider these five to be the most likely in your case:
- E. arvense - http://www.brc.ac.uk/plantatlas/index.php?q=node/2487
- E. hyemale - http://www.brc.ac.uk/plantatlas/index.php?q=node/1343
- E. pratense - http://www.brc.ac.uk/plantatlas/index.php?q=node/1345
- E. sylvaticum - http://www.brc.ac.uk/plantatlas/index.php?q=plant/equisetum-sylvaticum
- E. telmateia - http://www.brc.ac.uk/plantatlas/index.php?q=node/1348
The Plant Crib also lists the British species, and it explains which characters to consider in identifying the more-similar (ie. trickier) species
If you can't decide on a species, be aware that the issue is complicated by hybrids. Here are some examples:
If you do the species identification, please post here to let us know.
Many thanks for the identification information, I'll have a look at the literature and try to Id the species next time I am along the cycle trail
Troubling things are "common" names - Mare's tail in my mind is strictly the aquatic plant Hippuris although a lot of people do ?incorrectly call Equisetum species this.
The plant in your pictures is Equisetum arvense - the common or field horsetail which is probably the most variable and certainly the most common species. There are several things to look at when identifying them - does the green shoot have a cone on top? Then its always worth looking at the sheaths which are at the nodes of the stem - what shape and colour are the teeth, how many are there - then you can compare the length of the first segment of the branch with the length of the sheath - it's longer in arvense, much shorter in the most similar species palustre.