Interesting that you have a smart split relay set-up. Why ?
If you have certain types of voltage sensitive units they are designed to allow your leisure battery to pass charge to your starter battery.
Good idea ? Sometimes !
If your starter battery fails, as yours did, your leisure battery will try to assist it, as you suspect.
The problem is that leisure batteries are not designed to start vehicles, most have low CCA specifications.
Furthermore, the cabling from the leisure battery will not be designed to take starting current.
The voltage drop will, most likely, be excessive thereby not providing enough voltage or current to be able to crank the engine.
If your leisure battery is at the back of your vehicle, as mine is, the drop will be even more.
I have a CTEK MX5, and can recommend them.
You can even get leads to 'permanently fit to your battery which lowers the risk of accidents when connecting / disconnecting.
Your alternator should output close to 14v.
I use a cheap voltmeter in the cigarette socket to monitor voltages when on the move; effectively checks the alternator output and the battery becoming fully charged.
I move it to the rear leisure battery sockets when parked up.
Glad you sorted it.
If you do not use the vehicle much in winter, you must keep that starter battery fully charged.