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I'm looking for a (plastic cased) 12 to 15 way split load (80A
RCD/100A plain) preferably with a moveable division, but would also
consider separate RCD & plain units. Price matters but not nearly so
much as quality - ie I don't want to have to replace it again (ever -
if eternity exists).

many thanks for any advice/experience/views.

No opinions as to makes, but if you want maximum flexibility for any
changes
that you might make in the future, then consider installing a plain CU and
individual RCBOs or MCBs depending upon each individual circuit. This is
not a particularly cheap option, but one big advantage is that a leakage
fault in a circuit will take out only that circuit and not the entire
RCD-protected rail.

This is the way that I will go when I get round to replacing our CU this
year.

I was also thinking of doing this but I want rewireable/cartridge fuses for
the lighting circuits instead MCB/RCBO. Does anyone know of a CU with this
option?

I have always used Hager. I've been fitting them since I was an
apprentice. You can get all sorts of different CPD's - MCB's, RCBO's,
HRC's and all types of each. Top quality, good price.
You could even put all of your heating and lighting controllers in
there too along with a built in emergency light but that may be a
little excessive for what you want!