There is normally an overflow pipe from the top of the filler pipe which lets out excess fuel in the event of overfilling, fuel (in common with most liquids) also expands when it gets warmer so that is sometimes the cause of excess fuel 'leaking' out of the overflow pipe. Also when your car goes round a corner it leans and fuel can in theory reach the top of the pipe, if your filler cap is faulty it could leak slightly, is there any fuel outside of the filler cap, in the recessor on the bodywork? This was a problem with old Metro's and caused accidents as the fuel leaked onto the tyre. If its the filler cap a new one should be pretty cheap.
Is the nozzle normally right in when you fill it it? If not the cut off would be delayed resulting in it overfilling, also it cuts off for a reason and squeezing in an extra 27p is actually overfilling the tank. This sounds a likely cause as it was overflowing before you had left the garage.
If your'e not sure that overfilling/faulty cap is the cause I would get it looked at professionaly as the consequences could be disasterous, an independant garage could be more competant than a dealer with this kind of problem.
Mike