Now I've got the Sterling unit in and working, I thought I'd just to update you on the practical aspects of this installation....
1) On my Beta Greenline 35 there is a permanant 12v feed to the domestic alternator. This is lost when the Sterling unit is installed, resulting in the charge warning light not illuminating when the ignition is switched on. In that configuration it's also necessary to 'blip' the engine to get the system to fire up. The answer's to run a fused 12v feed from the ACC terminal on the ignition switch to the IGN input on the Sterling unit. Sterling recommend using the starter solenoid circuit, but then the charge warning light only comes on momentarily when the starter is cranked.
2) The wiring diagram in the Beta manual is not the same as the 'as installed' layout from the factory....
3) Does it work? Yes it works well and seems to do 'what it says on the tin'.
4) With the benefit of hindsight would I do it this way again? Probably not. I share the reservations of others in this discussion regarding the potential for taking down both alternators, but hey-ho, I've gone this route now....
5) How would I do it next time? Bigger domestic alternator and a digital regulator on this circuit only. Keep the starting circuit separate....
(Didn't realise that alternators, batteries and charging was such a discussed topic, but then we learn something every day.)