When do custody fees start when transferring an ISA to IG?

[Guest newbieguy]

I understand there is a £24 fee per quarter (Jan-Mar; Apr-Jun etc) for the ISA.  My question is why do these fees start when transferring an ISA from a different broker?  

For example - 

• I apply to make the transfer on 29 March - and the transfer completes in Apr (next quarter). 
• Will £24 be applied from the day I apply for the transfer (in Mar)  OR
• Will the £24 apply only from the day the transfer is completed (in Apr)

If it is the former, it seems silly to apply for a transfer that spans across 2 quarters, as I'd get charged £24 x 2 even though my transfer is not complete?

Thank you for clarifying.