Please test
Ok, still work in progress, but I have a working version.
If this solution is sufficiently robust, I may write a tutorial.
Here is the page,
http://whitehouse.forthwebsolutions.com/findus.php
the option to 'Get Directions from current location' is only visible when the browser window is 768px or lower (ie mobile devices)
Grateful if anyone is prepared to give it a go, and let me know if it works ok.
Thanks