Thanks for all your comments and advice.
Sorry - the info I gave wasn't accurate
The actual rusted area was less than approx. 1sq.m. Then they tore off approx. 2 more sq.ms of the tetextured paint on the roof, and after derusting they painted it. I guessed the measurements before, thinking the. Oat roof is 2m wide! Idiot) I thought it would be good for a few years.
I paid £600 for the job, but was trying to be fair by subtracting £200 for paint etc. I did not ask for a cheap job. I asked for a proper job, and paid the price they asked. Other boaters who have seen the rust have said I paid a lot of money for what the job entailed.
I had lots of maintenance work done there. My final bill was £2600.some of the other work left something to be desired, and they had to come back and redo the seals on the portholes ecause water started running in when it rained. That never happened BEFORE they did the work!
This boatyard is supposed to be reputable, do a good job for a fair price. It came recommended, and has many excellent reviews, though I suspect most of those are just for blacking.