The girlfriends house is fitted with an old Memera 21 consumer unit and
about 2 years ago one of the 5amp MCB's for the lighting tripped and
wouldn't reset (not sure if it was the circuit for the upstairs or
downstairs). She got someone to look at it who said the MCB was faulty and
as they couldn't get hold of another one connected both the up & downstairs
lighting on the one remaining 5amp lighting MCB. They don't make these
breakers anymore but I got hold of an identical one from the net (M6 5amp
51QEB Type 1).

No problem fitting the new breaker but I'm just a bit confused about the
wiring. There were 3 red live wires connected to the one working 5amp
lighting mcb. Through process of elimination I established which wire was
for the upstairs lights - and with just that one wire connected there is no
voltage showing at the other reaming 2 wires. However, if remove that wire
and connect one of the other 2 wires I get lighting for downstairs and also
mains voltage showing at the 3rd remaining wire (and vice versa, if I
connect just the 3rd wire I get power for downstairs lights and also mains
voltage showing at wire 2. I've connected the one wire that operates the
upstairs lights to the new 5amp mcb and the other 2 wires for downstairs
(which seem to be looped) connected both together to the existing 5amp mcb.
Just wanted to make sure if this is correct as I wasn't sure why 2 of the
wires power the downstairs and just one for upstairs?