Perhaps a little more info on the 'legality', Dave.
There are minimum amounts of fuel required before you can 'legally' push back from stand. These include 'contingency fuel' which is for 'unexpected' events. It is permissable to use some of this 'contingency' on the ground in the event of an unexpected delay waiting to take-off, but if you have, and you had loaded near the 'minimum' figure to start with, then you could not 'legally' depart again after a return to stand - there are some options on flt planning which might help here - but if faced with a 20 min + 45 min delay, plugging into a bowser seems to be eminently sensible to all of us.