Hi Roddy
If you want PWM pin PA6 (physical microcontroller leg 7) , the line should read
const int enable = 4; // motor pulse pin D4
PA7 would be D3. The PA reference is used if you are coding the low level AVR C, whereas Arduino abstracts the names further as digital or analog pin numbers.
The PA6 pin is addressable as either D4 or A6 in the Arduino IDE.
I suspect you want to check your other connections now too.
BTW I keep a print of that akafugu reference page in several places around my house. It's so useful !
Cheers ! Geoff