Hi,
I completed my PhD within 5 years (4 with the research, 1 to write-up). My university was quite strict that we had to complete within 3 years, as they didn't want PhD "Dinosaurs" clogging up their research offices.
Although we did have industrial partners, I did all of my research based at the University and I felt short-changed when other people had terms spent working at their sponsoring companies... some even got to work in some other countries (one of my work-colleagues spent 9 months based in the USA, while another spent 4 months in Burma, another 3 months in Germany etc.). However, after reading your story, I feel quite lucky as I can't fault my supervisors, and I'll be the first to admit I would never have successfully submitted without them (I had the same guys from start to finish).
You say your supervisors changed several times... was that your decision or not? I was always told that to split from your supervisors is basically the academic equivalent of a divorce, and should be avoided if ever possible. In fact, one lecturer I know moved from Swansea University to The University of Warwick, and he took his three PhD students with him (they got a "free" University upgrade, so they weren't complaining lol).
It sounds like your supervisors are not interested in your work... or they feel like they've been "lumbered" with you. To wait almost a year for feedback is unacceptable IMHO (the very least they could have acknlwedged your mails, but to completely ignore you for that time is just downright rude). I have experienced a "supervisor" who was like that (no where near to the extent you've described)... though luckily it was for a final year project and not a PhD, so I sought support elsewhere. Who did you raise your concerns to? Someone within your team or department, or at a more senior Admin level? As they've invested all this time and effort in to you, it's in the University's interests to ensure you can complete your write up.
Have you managed to get any papers published (ideally journals, but conferences are still OK)? If so, then that at least shows that the work you've done has some merit, even if the supervisors don't see that.
Just remember, the nice thing about a PhD is that if a particular part of it is getting to you, you can leave that part and work on another area...although this is probably a bit late for you're already in the write-up stage. I'm not sure how much a lawyer will be to you in this situation, and could be costly (unless you qualify for legal aid somehow). A lawyer consultation should be free, so they can obviously advise you what you can and can't do. You should also go to a Citizens Advice Bureau (if they have one in the country you're in)
Not sure if any of the above helps you but hope it works out for you anyway.