You can "leach" some of the plasticizer out of any line or particular bar of polymer clay if it's too sticky, as mentioned below (there are various ways to do that**). If too much
is removed though, the clay may be weaker after curing than it would have been or needs to be for certain usages.
They "causes" of sticky polymer clay are usually the brand or line of clay purchased since some are just stickier than others naturally (and sometimes there are just differences between colors in a line or just between inidividual bars when purchased), as well as having too much heat/friction applied to or around the clay.
(And btw, "softness" and 'stickiness" aren't the same though they may sometimes go together.)
All polymer clays have gone through formula changes over the years and all of them during the last couple of years because of new stringent rules for clays sold in the E.U. Those changes have generally made polymer clays softer and sometimes stickier. But some brands are definitely sticker or softer than others...here are some of the clays in general order of least soft/sticky to most (although there are always exceptions, and older formulations may be different for some):
...Kato Polyclay, FimoClassic (firm, not sticky)---best for those with hot hands, environments
...Premo, Cernit (medium-softness; "heat-responsive" which means they'll get soft more quickly than others with the same amount of handling, especially for those with hot hands or in warm conditions)
...FimoSoft (sticky and soft)
...Sculpey III, SuperSculpey (soft)
...BakeShop/CraftShop (softer and sticky)
...original Sculpey (very soft)http://glassattic.com/polymer/Characteristics.htm
And if you meant one of the sculpting clays by "Fimo" like Puppenfimo, or by others, they also have their varying characteristics for handling :http://glassattic.com/polymer/sculpture.htm
In addition to leaching, all polymer clays can be firmed up by cooling as you obviously know; that may affect stickiness at least some. Cooling can mean just resting for awhile to overnight, or putting briefly in frig or freezer, or instead cooling one's hands occasionally on an ice gel pack/etc while working.** If those things aren't working for you, I'd suspect you need to leach, or change brands, or learn to have a light touch when shaping the clay (not "work-and-work") the clay.
I wouldn't use the cool water
though, especially when using Fimo or Cernit clays since they contain more kaolin than some of the others and can get gummy when wet --that's why a powdery release is usually suggested for those brands when they're pressed into molds while other polymer clay lines can easily use powders or water/moisture. There's always the chance too when using water with raw clay that some of that moisture could get worked into
the clay depending on what you're doing, and create later problems.
Other brands and lines of polymer clay are available in the UK besides FimoSoft or FimoClassic (whichever you have) though, perhaps at retail places like Hobbycraft and The Range, and definitely by mail order from Sue Heaser's Polymer Clay Pit:http://www.polymerclaypit.co.uk/thepolymerclaypit.asp
and Emma Ralph's EJR Beads has at least the Fimos, as well as Cernit:http://www.ejrbeads.co.uk/shop/index.php?main_page=index&cPath=17&zenid=37983a79ada85934d2eea693505589c8
** click on Leaching
or on Cooling
HTH, and don't give up! There are almost always remedies or workarounds
. Exactly how are you using your clay though?--what are you making?