As I mentioned, I wish to use the secondary output of the Victron Phoenix, but I didn't give enough info. Sorry about that!
Here is a quote from the users manual:-
"Two outputs to charge two battery banks"
"The Multiplus features 2 outputs, of which 1 can carry the full output current (120 amps). The second output, limited to approximately 4 A and with a slightly lower output voltage, is intended to top up a starter battery."
So even if I should be putting 30 amps into the starter battery, the Victron apparently limits the current from the secondary output to about 4 amps, so a diode should be protected.
I also do not start the engine when connected to shore power.