Thanks for your help Bybell.
I used the system this morning to put 100 units on our Turkish mobile, and I have a receipt from the credit card company to show that Turkcell have taken the 16.5 YTL.
My son is in Turkey, with our Turkish mobile and I texted him to see if the units have been credited. He did have a text in Turkish, just moments before he got the text from me, but the units have not yet appeared in the balance - so I don't know what's going on.
I could not copy and paste the Turkish message as it disappeared just as I was about to highlight it for copying.
If the credits appear in the balance my son will text me to let me know - I will then let you all know.
I am wondering if you perhaps have to claim them in a similar way to claiming the bonus units that you get from the normal top-ups, but really, I would expect the balance to be increased in the same way as when you add units from a scratch card top-up.