Can I apportion gift aid income or staff HMRC contributions to the correct calendar month, irrespective of when the transaction occurs?

Yes, you can make this adjustment in the Adjustments screen.

Apportioning gift aid income would be recorded as Accounts Receivable, set up for each month (or copied from one month to another). HMRC contributions (PAYE, NIC etc.) would be Accounts Payable. In both scenarios, the adjustment is created by following these instructions and then reversed out in the month that the income is received or the expenditure is actually paid. 

Please note. there is no statutory requirement for you to make the adjustments referred to above as you do not need to account for these items within any one particular calendar month. However, it is a requirement to do this at year-end, where the sums involved are considered material to your charity,

To fully understand the Adjustments module in ExpensePlus, please view the module overview video here.