it depends
if you have a modern split load cu with space on the rcd side and a decently rated rcd (80A or more) then you buy a 45A mcb of the right brand clip it to the din rail in the rcd side screw it to the busbar wire the live to the mcb the neutral to the neutral bar for rcd cuircuits (if you aren't sure which is which follow the other wires for breakers on the rcd side)
if you have a modern rcd only cu with a rcd rated at 80A or more and a space then proceed as above
if you have a modern switch only CU with a switch rated at 80A or more then you can use a 45A rcbo (again fo the right brand) theese clip to the rail in the same way as mcbs but have a black wire you conenct to the neutral bar and a cream wire you conenct to the earth bar. live and neutral of the outgoing cable go to the rcbo and earth to the earth bar
if you have a modern split load cu with space on the non-rcd side and a main isolator rated at 80A or more then you can fit a rcbo on the non-rcd side as described above
note that some brnads don't make 45A breakers so you may need to spec your cable for being on a 50A one if 40A is insuffican't for your shower
if your cu does not match any of theese descriptions then you should use a seperate shower unit like this
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
PULL THE SERVICE 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
and put the service fuse back
the main switches in both cus should be off when handleing the service fuse to reduce the risk of arcing
feel free to post a picture of your cu