OK, OK, I think you've convinced me!
Here's a Wylex 10-way dual RCD CU for £58, with MCBs at £2.88. I think I will go with that.
That also gives me the option of having two lighting circuits, arghhh!
So, a Wylex dual RCD CU, with;
RCD-1:
32A main ring
40A shower
6A possible...