A friend of mine is an electrician, because he's qualified about as far as he can go he gets alot of extra work (he's employed by a large building firm during the day). Anyway, he fitted some extra sockets for his cousins friend who then didn't expect to pay because he was his mates cousin. Needless to say he kicked up a fuss and got paid what he was owed.

Don't be guilty mate, you did a job, you fixed the problem as quickly as you could. You travelled a long way (in UK terms ) to get there. I see no problem with charging for your time. It doesn't matter that it only took five minutes, if you've spent two hours travelling then it still counts. If he was just round the corner I'd revise the pricing downwards.