Hello,
The OV7620 is not compatible with this project. The OV7620 has a 27 MHz crystal source to clock the chip (since the chip delivers a higher resolution of 640 x 480), and this speed is too high to also clock the AVR mega8 synchronously with it. That is, in the current configuration...
If you wanted to add a flip-flop between the OV7620's external clock output line, and the crystal input line of the mega8, you would be clocking the mega8 with a 13.5 MHz signal which would work. You would be operating on a decimated frame though...and this would obviously require hardware changes.
You should be able to purchase the OV6620 from
http://www.electronics123.com , which is located in the US but most likely will ship to the UK.