Happened to me the other month, I quoted a job at the end of a day and was tired, said yeah it's X amount , did the first clean and thought this is gonna take a lot longer than normal , did the second clean and thought I cant b arsed for this much so said to the nice lady, I'm really sorry but iv underpriced your job, to my surprise she said- I told my husband you were doing it too cheap, how much will it be now? I said X amount , she said it's absolutely fine - happy days :-)
(To fill in the blanks I quoted £8, should have been £9 :-) )