Thanks for confirming what I thought, gentlemen.
The fan-heater as a fixed load is a good idea, and easy to implement, I can just 'borrow' the one from the workshop! The inverter is a 1600w Sterling Pro Combi Quasi sine wave jobbie of indeterminate vintage, so I'd feel a little wary of just running the heater until the alarm sounds. We have 5 off 110Ah 12v batteries, fitted in 2014!, so I'll probably run the heater for an hour and take Amps used and voltage readings from the BM-1, and decide how much longer to go from there. I know that the voltage will recover after stopping that kind of load, but at least I'll get a (very) rough idea of capacity and an indication of how soon the batts will need changing!
FYI - It's a holiday boat and we have 360 watts of solar and cruise for about 4 hours a day when we go cruising.Yeah, I know, bloody sunshine boaters! ?