I want to run a new cable from my consumer unit to pick up 2 CO alarms
and 4 smoke alarms:

Is a 1.5mm cable from a spare 16A MCB okay?

Yes.

Does the cable need to run from alarm to alarm (i.e. radial) or can I
run a single cable to the furthest one and spur off at 4 points for the
other? (the later would use much less cable)
If I did the latter, would it cause problems?

It is normal for the smoke alarms to need a 3&E interconnect. You normally
achieve this by running the main supply to the nearest alarm. The other
alarms are then connected to each other from the first one using the 3 core
cable (such as that intended for 2 way lighting circuits), which usually
consists of a live, neutral and signal core (plus the earth, of course). You
can run the cable in any manner that is convenient, whether star or daisy
chained. If the alarms genuinely use 2 cores of the interconnect as live and
neutral, then the initial (T&E) supply line can go straight to a star
topography junction box.

If the alarms are of different brands, then the interconnect probably won't
work/will destroy the alarms. You should take them back/discard them and buy
same branded linked alarms from a single manufacturer.

Christian.