I'll add my recent experience with this as it may help others with the same problem. I thought I might be able to fit three new panels myself, but also wanted to replace the seals on the other windows. Before I ordered panels, I did a dry run to see how difficult it might be. I followed the advice given in this thread and removed the whole window by drilling out the rivets on the hinges. I also had to remove the hinges completely from the window as two of the 4 screws holding the unit together were underneath. After a bit of struggling I got the 4 badly corroded screws out, but then I just could not get the frame apart. I was starting to think they had used some kind of epoxy adhesive as it wouldn't budge no matter how I tried to prise the sides apart. In my struggling I managed to crack the glass (thankfully on a pane that needed replacing anyway). I decided to call it quits, refitted the window and called a local glazier!
They came this week and replaced the three panels. Interestingly they left the windows in situ, only removed the bottom screws and prised the sides outwards until they could remove the bottom bar and pane. (Team of two so one could support the weight of the window while they did this). As they had used this method they didn't fit the factory fitted U-channel seal, but just used normal seals once the pane was fitted. The cost of the panel and fitting was about £220 for each window and they took just over an hour for each one. I thought that was very reasonable and I'm really happy with the finished job.
Incidentally I had bought a length of R0929 Patio Door U Channel Gasket Seal (when I thought I might be able to this myself). This seal which is 24mm wide and the right shape, doesn't fit - it's too narrow.
The glazier said these windows are a bit of a challenge, especially in coastal areas where the screws corrode and won't come out, or the heads just shear off. I've decide just to leave the seals as they are, on the remaining windows - the cost, and risk of damaging the glass in the process is just too great.
I hope that helps anyone with the same issue.