I don't use Nationwide but a cash ISA account is normally an account for savings you don't take out unless it's an "emergency." Some cash ISA's are operable online but some aren't - even if it shows up in your accounts online. You might be better opening another savings account online that gives better access to cash than your e-saver. You can transfer cash to your ISA online up to the deadline but perhaps do it by 4th April. Remember that any stocks and shares ISA you've invested in this year also counts towards your total ISA annual allowance.
P.S. I trust you have decent up-to-date antivirus and antispyware on you pc.