Just an aside is that often the alternator output is designed to droop with temperature, so as to cloer follow the battery temperature coeficient, so the cold output voltage will be 14V4 and drop to 13V8 or lower as the engine heats up, so as to not overcharge the battery, which these days has a limited overcharge capacity due to there often being no easy way to introduce fill up water in them, thus if the hot charge voltage is too high they boil off water as hydrogen and oxygen gas, depleting the electrolyte.
i notice this on my car, cold the battery voltage is 14V4, dropping to 13V5 when hot after a long (30km) drive, measured with a meter that is effectively across the battery terminals. Turn on the lights and the droop is to 13V0 after a while, from that extra 30A draw of the lights. Turn on the rear demister and the loom droops 0.4V, as that is my tap off point, and the 0V4 is the drop in the thick feed wire from the fuse box, however compensating for that the battery does stay at 13V0.
Best for Simon though is to either spec 2 alternators ( if space is available to put one in there, though whoever has to maintain that vehicle will forever curse you, your ancestors and give speculation as to your state of mind in doing such) and make them lower power units, arranged to feed either a common bus for the battery, or better yet 2 batteries, one solely for engine and accessory power and not for use with any engine off loading, and the other battery and charge sytem for any loads that are engine off or which are not engine related. As well I really would recommend a pair of electric cooling fans for the engine bay, one running off a thermal switch that operates at 70C, and the other at 30C, so that you have cooling air at all times. Switches probably best places high up in the bay, with air inlet near the bottom one side, and exit high up on the other, Screen both inlet and outlet against debris and rocks, using a heavy duty screen on both, so you keep stones and sand out of the fans.