I just replaced the battery on my polar watch and 'error' and the number 6 is on the screen. Nothing i do will unfreeze this or change it. Have already tried to take the battery out and put back in

You probably need to "short out" the battery before you can set it.
To do this you need to remove the caseback and apply a tweezers to the battery and the "AC".
The AC is usually labeled and it looks like a little gold cricle near the battery in the watch. (I realize there may be more than one gold colored small circle in there, and if it's not labeled it will take trial and error.)
With the tweezers, or paperclip would work, touch the AC and the battery at the same time and hold for a few seconds, then release and repeat once more. This should get your watch reset and ready for you to set.

In my polar rs400, after the battery change, emerge the error 255, and the monitor was useless, the system was recovered by conecting with the PC via IR communications with the data trans, the program in the PC was the PolarProtrainer, after that, the error was eliminated, the system initialized and the moitor like new.