Default units
I read in the api documentation that when coordinates are used, default units are always metric.
(I am using coordinates as the postcode isn't accurate enough).
I read you can explicitly set units as metric or imperial, but wasn't sure where to put it.
OK found it.
If anyone else has this issue, add a line to the block starting directionsDisplay.setPanel (.... -> (around line 183 in the map.php page)
replace
travelMode: google.maps.TravelMode.DRIVING
with
travelMode: google.maps.TravelMode.DRIVING,
unitSystem: google.maps.DirectionsUnitSystem.IMPERIAL