As Deano says it's not right for anyone to rip someone off and 400 quid for 1m2 sounds ridiculous, but we also don't know the full facts and have no idea of the prep involved or anything about the situation. So many times i have agreed with a customer that a job will take x amount of days and be x price based on the discussion i have had with them IE: if skirts are removed, carpet removed, area cleared etc etc only to turn up and find out that they have not kept there end of the bargain and have then had to spend several hours doing it myself before i can even get on with what it was i priced to do, it's a rarity i charge for this but it is something i find a complete liberty on the customers front.
I did a 4 m2 kitchen in metro's earlier on this month and it took me...... First day 8am until 6pm, second day 8am until 8:30 pm (22 hours) cos the job was not as the customer said it was. I didn't charge any extra for the extra work i done as i knew it would create a bad feeling but at the end of the day the customer had a lovely job that involved a ton of work at a good price £250 and i walked away knackered, peed off and feeling like i had been taken for ride (obviously there was more to this job than you are hearing, i had mentioned the extra work to customer but i could tell it would just undo all the good work i had done if i then charged more).
And each post that op is leaving more facts are coming out so it might be a case of them not being straight with the plumber in the first place and the plumber charging 2 days rate plus materials...... Who knows. Like i say if i could get 400 quid for a day on my jobs i would charge it and lose no sleep about it.