If the red light on the pull cord isolator switch lights all the time it means you have cross-connected the input (Black & Red) with the output black and red - not an ideal situation but easy to rectify and not immediately dangerous.
If the input red and black are connected properly - on the input side - the tell-tale safety neon will go on and off as you pull the cord and turn the switch on and off - the output to the shower need not be connected. The Neon is an excellent aid in discovery...
As you have no neon illuminating in the pulll-cord switch it appears that there is no power being supplied to the switch - the wiring or the breaker should be suspect.
When you go to the shop to buy a new Circuit Breaker invest in a Mains Tester screwdriver.
Your text, "changed the wires round ie two reds and two blacks" - I hope you have not connected two reds on one side of the switch and two blacks on the other. If so that would cause the fault you describe.
You may also have a fault with the shower itself - hope not...
Peccavi
May 2009