I did one of the windows on Tranquil Breeze where we had a leak. It had been caused by something hitting one of the vertical mullions, but instead of doing the job properly, a previous owner had simply gobbed a load of mastic in, which needless to say didn't work.
I had to get a new section of mullion cut, took the window out (it had to be the biggest one at over 8' long), dismantled the frame, reassembled it with the new mullion and rebedded it on marine frame sealant. Bob's your uncle no more leak and only a days work.
£1500 does sound steep, but then if you are paying for workshop space then I suppose that all adds up.