From the BRITTEN-NORMAN ISLANDER A.R.B. APPROVED FLIGHT MANUAL FMI7
Part of the PRE-STARTING CHECKS (INTERNAL)
There is another section where they are dealing with the unfeathering an engine (re-starting an engine in flight where there can be no external supply) and the proceedure has:
8. Select External Supplies switch EXTERNAL SUPPLY
The after the engine has started:
15. Select External Supplies switch OFF/STARTER ISOL
I take this to mean that if you want to start the engine the External Supply switch needs to be in the External Supply position or as the switch Tool-tips have it External Power set to On then after the engine has started External Power set to Off.
Safe flying, sPK
PS - It’s a little academic as there are lots of other problems and the have not been attended to yet but I live in hope the will be 