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Hi Nicola, how's the weather in Basingstoke? Oh, I'm sorry to hear that (lol).
There are numerous causes for this common (yet absurdly irritating) phenomenon. Usually, however, it is simply due to a loose wire and can often be fixed by pushing the wire towards the earpiece and, if needs be, affixing it into place with a little electrical tape, super glue, or some other adhesive.
Sometimes, if the headphones have an inline volume wheel, that can be the cause. That one is a little trickier to fix, but you could always try the 'wire trick' mentioned above and see if it works. If not, then open the volume wheel and re-solder the wires into place (be warned, this will invalidate most warranties, so if the 'phones are still covered, just send 'em back and get replacements).
To avoid stuff like this happening in the future, it is advisable to wrap your cables carefully and to avoid stressing your walkie Talkie Headsets. No, I don't mean that you should give Motorhead a rest and only play floaty, soothing New-Agey music on them, I mean that you shouldn't have them in your back pocket when you sit down and you should remove them carefully from your ears after use (this may sound obvious, but you'd be surprised how many people just rip 'em out).
Another thing to look out for is the jack, if the cable is fraying/wearing around the jack, then that can also be a problem. Fortunately, like so many things in life, a bit of electrical tape can really come in useful, just ensure that all the copper wiring is tightly bound and it should return to normal usage in no time.
Sometimes, however, it is simply a sign that the headphones are knackered and no amount of clever tinkering can fix them. Usually, in these cases, the problem is internal. This particular variant on the problem also attacks headphones of any price range, be they Poundland specials or your year old Sennheiser Eargasm series. I'm reminded of a Shakespeare quote from Hamlet, something about a king and the guts of a beggar, but I can't be bothered to look the whole thing up right now. You get the point I'm trying to make though, everything dies eventually, no matter how much it cost you.