Thank you Ckiick. I think I am getting to grips with it now. Please let me know if the following is correct.
In the Atmel Datasheet (Doc 8161, Ch 11) there is a list of 26 interrupt vectors. If I want to use any of them I MUST use ISR().
Ch 12 of the datasheet deals with external interrupts and I MUST use attachInterrupt() to deal with them.
@AWOL, perhaps you would be kind enough to RTFM and tell me where it explains ISR()?
...R