I did my cooper D after the main dealer could not get the Eolys kit as it was on back order, from dealer a replacement 1.1ltr bag is used.
Jack up rear of car, use ramps or axle stands as a minimum, 4 post ramp much better,
Remove rear section of plastic floor cover on off side, undo to about drivers door it will then drop enough to gain reasonable access.
I removed negative battery terminal for good measure but not sure this is needed.
Just in front of the rear off side wheel is a black suitcase type box, with fuel type fittings.
Remove the fuel type fittings and undo the bolt underneath the box, be careful as mine started to spin the top nut, which you cannot get to hold.
(i ended up hack sawing the bolt and then replaced with new nut and bolt)
When its off it looks like this,
If yours is leaking its going to be a new bag, you can see where mine is just starting to show signs of wear.
Undo the bag from the green fitting, and either replace or re-fill.
Mine filled up with 1ltr not the 1,1ltr mini suggest i will keep an eye on the warning and change early, next time a new bag will be fitted as a good measure, i used a kit off Ebay £35.00 for pug, citroen etc all the same.
You cannot re-set through the cars own method it has to be main dealer, or an auto electrician it still took him 20mins to find the re-set and untill it has been re-set the pump turns off, that's once 0 miles has been reached, i have now done 20000m since doing and its fine.
I would say if you can afford the £200 or so mini want to do it then let them do it, i was stuck doing 700m a week and already empty so was worried about the DPF blocking up, if your above average ability with the spanners and fancy a challenge do it but don't blame me afterwards.
This is a guide how i did this and will take no responsibility for your actions, hope it helps though either way.
PPS you cannot back fill the bag by removing the fuel type fitting on the outside of the suitcase box while its still fitted to the car, don't know why maybe the pump stops it, but i did try.