adding a shower CU is not rocket science but you must work carefully and make the connections well
remember bad connections on a 50A cuircuit can be bad enough and this is 100A wiring so take care
this is a shower CU
http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=05435&id=69659(you can change the mcb to a 40A if suitable)
you will need to henly block it into the tails this is done as follows
fit a double pole henly block to the fuseboard
http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=05950&id=14850 turn off the main switches on both the existing CU and the new CU
PULL THE SERVICE FUSE (TAKE CARE SOME OLDER SERVICE FUSE UNITS LEAVE AN EXPOSED LIVE WHEN YOU PULL THE FUSE)
remove the tails from the meter to the existing cu
use fresh 25mm red and black double insulated tails to rewire the cu's
connect meter to henly block
conenct henly block to main cu
connect henly block to shower cu
conenct the earth from the new CU to your main earth terminal
http://tlc-direct.co.uk/Products/TLEB8.htmluseing 16mm earth cable if you do not have one install it between the main earth and the CU useing 16mm cable from the main eart to the main earth terminal and from the main earth terminal to the cu and move main equipotential bonds to it
put the service fuse back
turn back on main switches