Starting A Letter

Hello, I know it is advisable to start a letter with the name of the person you are writing to (ie Mr Jenkins), but I'm not to sure what to put in this instance.

I will be writing a letter to work about something I can't go into at the moment (pretty long I guess), but currently the general store manager is off sick but due to return in a week or two times. The deputy store manager is naturally the one in charge now while she's off sick.

I'm not sure who I should address the letter to, the GSM or the DSM? Or is it advisable in this instance to use one that covers both such as 'To Whom It May Concern'? It is possible that the letter may get sent to head office or something like that but I'm not sure as I'll be handing it to the DSM or is she's no longer sick, the GSM.

Thankyou, Ashley

[Anglika]

Address it to the DSM as she is the one in charge at present.