you shoudn't have to turn off the mains..most tritons will have a threaded connection at the hose end,which simply screws off the water outlet.Provided your shower is OFF,you can simply unscrew the old hose,and screw in the new.Check that the new fitting has a rubber sealing washer...don't reuse the old one.B&Q sell spurious replacements,aswell as the genuine triton parts.Good luck.
Chris, September 2007