String conatenation will at least be part of your problem:
StrSql = "UPDATE TBL_StockEntry SET TBL_StockEntry.[Date Booked In] = Date(), TBL_StockEntry.[State] = me.combobox1.value " & _
"WHERE (((TBL_StockEntry.StockID)=" & StrId & "));"
Should read:
StrSql = "UPDATE TBL_StockEntry SET...