My problem just got a whole lot bigger!
In my haste to follow your last instruction I left the 'enable' wire disconnected from the Attiny48. So in theory without that pulse the motor won't run... guess what.. it still runs! So now I'm totally clueless. The L293D needs Enable 1 (pulsing to control speed) and either OUTPUT 1 or 2 HIGH to drive the motor.
Somehow or other I don't need to ENABLE and it still bloody runs.
Feeling totally stupid and lost now!