Hi Jack,
Yes you are absolutely right. there seems more support for the 1284p than it's smaller brother. I just saw the 644p advertised on a suppliers site advertised as a go between the 328 and 2560 so thought I would give it a try. but in hindsight should have opted to get some 1284's which I have now done.
After reading the datasheets (most of which went over my head) I realised that INT2 is not enabled by default and you need to set the port up. even then it only responds to a changing edge not a rising or falling like INT0 and INT1 (Can't find the web pages where I read that 
)
I'll certainly have a go with your code once I get Studio working lol - Atmel support sent me a patch today but that won't even install so the issue is on giong. I want to get more into this type of programming as I can see some advantages - like it doesn't limit you to using Arduino IDE (which is still a great tool.
Thanks for your help and it has been great chatting with you.
All the Best Steve