Feed for Shower CU

[mainframe]

I have recently moved house and am putting in a new electric shower
(9.5KW). I am fitting a shower control unit (50A MCB 30mA RCD) as there is no room in my consumer unit. I have read in this forum that the best way to do it is to use a henly block and feed the existing CU and new shower CU from it. But my supply comes via main fuse through meter then through a 80A 100mA RCD and then to the main CU. My question is which side of the 80A 100mA RCD should I split the supply ie position the henly block?

 

[ban-all-sheds]

Before it, i.e. the tails from the meter go in to the henley block, and then two sets come out, one to the existing CU, and one to the new CU.

What are you going to do about isolating the supply?

 

[mainframe]

Thanks for the reply
Looks like I'll have to ask those friendly folks at Northern Ireland Electricity if I can break the seals and pull the main fuse!
Do the Electric companys charge you to reseal fuses meters etc providing they'll let you break it in the first place?
Although For NIE this would be rubbing salt into the wound due to their already high costs! comapred to you folks over the pond!