undergrads bring in more cash (non-overseas MA and PhD fees are often much lower than the annual undergrad 9k) and then there's the national student survey thing...

i had a change of supervisor too for similarly problematic reasons and the whole thing wasn't pretty but eventually turned out ok. had then a supervisor who knew nothing about my subject and had to stick with the less horrible one, so like you had to decide who of them is more awful. you have to see how this goes, it's always very tricky area to navigate, this kind of thing. but it can work out. it's also good to look at the formal side, there must be some kind of formal guidebook which says how much supervision you get, how this all is supposed to look like and if they're not doing it by the book this is something to mention at the meeting.... in any event it's good to make a list of things you feel that are going wrong... you're even doing their work, supervising that ma student. maybe talk to the student's guilt before that meeting to make sure you've all your ducks in a row. they can also advise you on what are reasonable expectations and what not. unfortunately, prepare yourself that things can get nasty.
as you indicated you have a longterm condition it's worth looking into any kind of extra support you can get, extensions etc. don't freak out of the time limit just yet, you can still add on an extension which you very likely will get for medical reasons. those extensions don't have to be short. 14 months is still a long time. plus, say, add on 6 months extension.
make a realistic plan on how to address/change your research so it can work out.
it's worth trying to find out why that other person changed the supervisor.

good luck with all.

So I had my meeting today and tried to raise some of the issues. My overarching feeling is that a lot of the issues were simply pinned on me - no acknowledgement that things in the situation/project could be contributing.

I was interrupted mid-sentence with the suggestion that I should go on holiday. I think the worst time to go is when stuck - just increases the chance of not wanting to come back, and prolongs actually making any smidge of progress.

A couple of thoughts...may or may not be helpful but I will put them out there anyway. It really sounds like second year slump to me.
I went through the part time version of it which lasted just under two years and basically thought several times a week, 'I should just give up'. I even had plans for giving up, which I discussed with family and friends but in the end, feeling like I was going to let my initial survey respondents down and the ethical committee kept me going-who knows why I felt so responsible to them but I did and so I limped along through the slump very slowly.

I'm lucky enough to have a great supervisor and a very busy but nice when I see her secondary supervisor, but while I really loved my Masters supervisor, we had real difficulties due to distance, different working styles and time lapses which nearly drove me insane at one point (well I exaggerate but it felt pretty bad at the time). In the end, I just came to a sort of conclusion that 'it was what it was' and I just had to bear it and keep on going. Both with Masters issues and the second year PhD slump.

(@Just an aside, the stats I am working with (not with my study but with in my workplace) say that one in five people will have a significant mental health issue at some point in their lives and I imagine that a PhD is enough to drive anyone to drink, so perhaps there are more who see counsellors than we admit to...)

My experience of PhDs are that progress is really uneven. You seem to be doing nothing for ages and then bam-you make a leap and there is some discernible progress and you have reached a milestone without even really knowing how you got there except through misery and tears.

Finally, (sorry long post) but while you are at this impasse, are you able to do some administrative type tasks, or improve the lit review or similar, so that you feel like you are doing something during these times when all the data, ideas and everything just seems to be swirling around in a blurry limbo. (I promise you, this happens for all of us in stages during this process). Best of luck, it is a tough time and a tough gig. Hang in there is my opinion. :)