Well, to tie this one off in the end, I decided given I have mismatched units, that repairing it, only for it to fail potentially in the future due to overloading the low power diodes, would be somewhat foolish, and just be something to worry about too much in the future
SO.... A plan was hatched, it involved 2*24V 100A relays, and one 24V 10A relay, and a bit of surgery on the board....
I figured that if the low power diodes didn't like switching the later relay, that trying to switch 2 100A relays directly probably wouldn't be too sensible. So, I only use the low power output to switch the small, 20A relay, current draw on this is likely to be less than 100mA, so nothing to worry about in my book. The 'power in' to this relay is taken from the high power generator output, and the 'power out' feeds into the two 100A relays coils, the other terminals of which are of course grounded.
The 100A relays are fed by 2 bolted in bars (M6 studs), and the output goes to the jumper wire which leads to T1, and the vehicle batteries output, which both needed opening out a smidge, again to M6. The diode wires for these two terminals were re-crimped with suitable ring terminals and put in place.
There was however a problem, T6 is fed from T2, via a 100 Ohm resistor, as discussed above, and leaving it in circuit for the small relay would mean I ended up with exactly the same problem, the relay not firing! So... I cut the bent plate/diode mount into two, between the first and second diode, ground it thinner, and turned it around, then connected T2 to this new terminal with a simple jumper wire with two ring terminals on it, omitting the resistor. The resistor still went to the 'other' T6 terminal, so that circuit remains identical. The reason for this malarky is to ensure the polarity protection remains in place.
People may wonder why two 100A relays.... well firstly, that is how the BCK108 does it, there are two sets of contacts, and they are not joined unless the engine is running (charge light out). Otherwise you could flatten both the radio batteries and the vehicle batteries meaning you couldn't escape when being shot at! Also, you have to design it with the POTENTIAL for one of the circuits drawing all 90A from the generator, hence 2*100A relays, and not 2*50A.
I've attached a VERY rough sketch of what I did, modified from your diagram, Clive.
It was a tight fit with everything in there, I ended up removing the mounting tabs from the relays, then remove the case from the relays, bolt this through the board, and then refit the relay innards to the empty boxes.
This may be useful for someone else, I don't believe it a hack, if I had a photo I would post it, but someone buttoned it all up forgetting to take a photo(!) It certainly works, 28.3V at the dashboard, and the same at the radio battery terminals, around 2 seconds after starting.
Thanks for your help, feel free to steal the circuitry if you feel valuable.