I'm somewhere around Daniel's definition of third path. It feels like the top half of the felt- sense of my body has dissolved. And not in a pleasant way. 

is this normal? Part of the process? 

I guess with mental health issues (anxiety/depression) I shouldn't have got involved in intensive meditation- but here I am....

Sounds pretty normal to me. The felt sense of the body definitely fluctuates, more so with intensive practice. If you are feeling anxious then you can investigate the physical sensations of anxiety as well, which helps take the focus/energy away from the mental freakout.

Here's another thing for anxiety you can try which sounds stupid, but has worked for me in the past. Say out loud in the most ridiculous high-pitched voice you can something silly like 'oh no, I'm having a complete freak out, the world is coming to an end and I haven't even had lunch yet'. By making fun of it you take the serious energy away from the anxiety and deflate it. You can actually end up laughing and in a strange way enjoying the anxiety trip!

For meditation-induced undesired sensations, I have found moving around in whatever way is accessible helps: a walk, yoga, calisthenics, etc. It can also help to eat and to drink. Basically, I put my body to work doing the things it normally likes doing until it gets the hang of processing sensations again. Gently, of course. The worse thing is to sit still in a room with no stimulus. 

Happily, after more practice, this has now developed into a ball of solid, black, infinite pain in the centre of my back that simply won't shift however I try to vipasanaise it. 

i f***ing love 'peaceful' meditation. I'm so glad the NHS introduced me to it. 

Heed the warnings in MCTB. Do not get involved in this if you have preexisting mental health issues. 

Change your antidepressant. Change your doctor. Take up stamp collecting. Make some new friends. 

I really hope this is reobservation and not the 3c's

Robin, please check this organization out - you may be able to find help there: https://www.cheetahhouse.org/get-help

Seriously, if you suffer from mental health issues, make sure you have a solid foundation first, before you keep going with intense practice. I did not suffer from mental health issues before I started with this path but a pretty bad meditation experience that happened a year ago let's me still suffer intensly, to the point where I feel suicidal and loosing my shit day after day. 
Start to really rely on your intuition and back off from practice if needed - don't have a "I need to rush through this" kind of mindset and throw out every possibility of having attained even first path. (I'm not saying you are pre path or whatever - but you'll know when you get to a point of total dysfunctionality)

I think I'm about 'third path' - not that that's helping me much. Lol. 

Robin, do you have a therapist you can talk to about your current symptoms? Also, what makes you believe you're in the neighborhood of MCTB third path? My suggestion mirrors that of Oatmilk - slow down, take a breather. Your mental and physical health is most important.
 

My experience is largely observerless and agentless - I've been in this game a loooong time - so that's why I think I'm around third path. Not that this is helping me at the moment though. I just thought that this would all get more subtle and less destabilising as you progressed. 

i have a therapist but I'm not sure she really understands what 'meditation' is never mind the progress of insight.....

shouldn't I keep practicing if I am in reobservation though? It kinda feels like I'm out of sync with reality or something and something needs to sync up

If I were in your position, I would probably contact Daniel to ask for qualified guidance. He does it for free and has helped lots of people in spiritual difficulties. I personally find him very good to talk to. You can find his email address om his webside https://www.integrateddaniel.info/contact where he also describes the terms for what help he can offer. 

I agree with Linda and suggest you contact Daniel Ingram directly. I suspect, based on the comment above, that you're misdiagnosing yourself in several ways and that he can help you with most of your issues.