So after a frustrating hunt for my dcdc issue I got it resolved today. I suspect that i got misled by there being two issues. The first i think was a poor connection in my connector to the charger. This misled me into thinking the unit was faulty. The replacement charger definitely had an intermittent fault on the same connector but on the charger side. Took me a few install/reinstall to find all this. Decided the connectors could be deleted, and soldered my own loom to the wiring - works now
That charger can output some 12v charge when it wants. Like 50amps.
The second issue was the ABS light on and numerous fault codes. The ABS pump is a known issue on the e46 so sent if off for refurb. They reported they couldn't find a fault but suggested it could be intermittent as the codes i was seeing were consistent with the known issue. So i asked them to refurb it anyway (i think its a solder reflow job). Once refitted my faults had gone but i had some new ones (pressure sensors, brake fluid level).
So spent way too long building a laptop with a 32bit Window 10 and loaded INPA, and managed to clear the codes. All i'm left with now is 'no can comms to dme', which is because the DME (ECU) is unplugged. Don't think the mot tester will care about that, but i expect with some research i can spoof the DME to the ABS unit anyway.
In parallel to all this, my handy mate has decoded all the e46 dash signals and is building up a custom pcb. Not as easy as it sounds as its an odd mixture of can, hard wiring and analogue resistance levels (the e46 had 2 fuel tanks and the dash averages them together with damping), but i think now we can control everything, so fuel will be an approx SOC, temp will be temp, MPG gauge will be amps etc
So this weekend, another road test and some logging and shunt calibration i hope.