Hi Schrodingers
Firstly so sorry to hear that you are in so much pain and feeling so frightened and low.
I didn't have any big health problems in my pregnancy's but even the 'normal' pains of late pregnancy were difficult to deal with so well done for getting this far 
I suffered from fairly severe antenatal depression, it was like a light went 'off' in my head at about 7 or 8 weeks pregnancy and alrthough I was never suicidal I had every black and negative thought going every single day until my babies were born! Once the birth was over though the light went back on (thank goodness).
I did discuss my feelings with my MW and we agreed that if at any time i felt unable to go on with no meds then she would refer me for an urgent appointment and we would look at the possibility of antidepressants. As you are now so far along in your pregnancy i am fairly sure therer would be very little risk to your baby (and a psychiatrist would be able to advise you of which meds had a good safty profile in pregnancy etc). Would you consider this?
It is awful that you seem to have 'fallen through the net' so far but my two bits od advise would be
one- make an appt with your GP tomorrow, tell them that you are now feeling suicidal and make it clear that it is mainly the pain and fear caused by such inconsitent messages and advise. Ask for an immediate referral to the local Community mental health team. In the area I work a pregnant woman having suicidal thought would be a huge priority.
two- there is in every area a 'mental health crisis team' who you can contact yourself, any time of day or night. They can talk to you over the phone or come and see you at home, if you ring NHS direct they shoyuld be able to give you the number for your area. They are a good resource as they can fast track you into services as well as offering support as and when its needed.
Also, what support do you have personally speaking, apart from struggling with physical pain and depression pregnancy is such a worrying and (sometimes) lonely time? You need the company and support of other expectant mums!! Although you are under consultant care you should still surely have a named midwife or at least a link team that could maybe put you in touch with a local group?
Sending you a virtual hug, soon you will be a mum and even though it wont makew the HMS go away everything will be so much brighter wheh you meet your baby.....