The percentage reading of BM2 is meant to give rough estimate of SOC of a battery being at rest for hours.
I would suggest let the battery be undisturbed over night and see the voltage reading then. Voltage of lead acid batteries sag even under small loads but bounces back at rest (up to 12 hours after disconnecting the load).
OK, it was left undisturbed overnight. I checked the BM2 last thing at night and this morning. Any suggestions please? The fuses are pulled from the Eber loom. I guess next step is physically disconnect the Eber loom from the leisure battery.
The BM2 suggested the battery recovered by 20% then overnight lost 10% again. Damn this is frustrating.