Hi Gavin,
I just wanted to say thanks for the excellent work on this.
I had a thought about dealing with the tile number restriction, and would potentially get you a few donations to this worthy project!
How about developing a 'premium' kml that could be modified to use a different API key. ie modify your server code to read the API key from the KML file. That way, a user could obtain the KML from you (for a small fee

), modify it to contain their own API key (subject to registering at OS), and voila - 30,000 tiles per day for that one user.
Once again, thanks for an incredibly handy utility. I use it for locating public rights of way when geocaching, and it was the handiest tool ever! I was devastated when it stopped working

.
Cheers,
Chad