Q: In app purchases

See:

http://support.apple.com/kb/HT1574

Once you redeem a gift card the entire value is added to your iTunes account. You do not have to do any else to purchase new items except purchase them.  When the remaining balance is not sufficient iTunes will either say you nedd to add a valid payment method or use a cred card automatically if you have one in the acccount.

I have £15 on my iTunes account and I'd like to make an in-app purchase for £23.99. Does it mean my card will only be charged £8.99?

Yes. Tax will also be subtracted from the card if the purchases are taxed.

Thank you for your rapid response.

Before I go ahead I'd like to know if ill be notified about available funding sources during this transaction?

Thanks.

You will only be informed if you do not have sufficient funds for the purchase. If you do not have the funds I think that the message will say check your payment method or similar. It will not way insufficient funds.