Definitely justified, particularly if you use your car a fair bit at weekend or travel to other places. Also justified because if you're doing a lot of mileage you'd want a slightly bigger, more comfortable car which would impact on economy rather than a 1.2 litre super mini. Also for motorway journeys, a TD 306 is more relaxing than a petrol 306 because you seem to waft along on the torque, so you feel less frenetic.
I used to do the journey in a 323i (80 miles each day) and weekend driving + trips away and was spending over £100 a week on average in petrol.