it is horrible, I know exactly what you mean - I'm 29 weeks and have really bad pain, mainly at front of pelvis and in legs, but since this is my first baby I don't have to look after another child, which must be really hard for you! I also sometimes just cannot get out of the house, or walk 10 mins to the shops and feel exhausted and almost like I've got flu as well as all the pain.
My first visit to hospital physio just got me a belt and some exercises and the message that there wasn't much they could do, but after a couple of weeks I rang the physio back (fortunately she turned out to be really nice and sympathetic, which I'm sure isn't always the case) and begged for another appointment, and since then I have been given weekly physio appointments, crutches (depressing but worth a try), and leg massages which do help a bit more, even if it's only because I feel like someone is listening. If the physio at your local hospital is unhelpful, maybe try being really firm with your GP or midwife and insisting on seeing another one at a different hospital? or finding out if there's an osteopath that you can see - not sure if this is available on NHS, but if private and it works, it might be worth the money, if you can afford it. Your GP/midwife should also realise that this is making you depressed, which is not surprising. My boss said 'hmm, i think you're depressed', and I did think 'of course I am, you idiot'. It doesn't mean you were depressed to start with, but facing weeks of pain and immobility and being disabled, when you're normally fine, is very depressing. I have taken some time off sick because of the pain and lack of mobility, and finally realised this week that since I'm 29 weeks, I can and should start maternity leave early, even though I really didn't want to, and it means less maternity pay/time off after the baby gets here.
some other things that have helped me a bit are getting co-codamol from GP (I had to ask twice, first time the GP I saw said no, not in pregnancy, but the next week I saw a different GP at the same practice and he was fine with it); pregnancy yoga class, having explained about the SPD to the teacher before so she could tell me which exercises I can and can't do; ice packs held over the most painful areas (my crotch - thank god I'm not a work now as I don't think that would go down well); aquanatal classes; or just going to the pool and walking/paddling around, not proper swimming; warm baths; not sitting on chairs - either lying down propped up with cushions or sitting on a gym/yoga ball, which cost about £15 on amazon; using a long maternity pillow in bed to put between legs.
I also tried acupuncture this week, which seemed to make it worse, although other people say it has helped them. This was a private appointment, about £50 and way too expensive (especially if I'd ended up getting it every week), but I've just spoken to my midwife who says that the hospital I'm booked into has an NHS midwife trained in acupuncture, so she's putting me down for an appointment with her...if it comes through, I will give it another try.
All of this took a lot of persistence in asking for - really felt fobbed off the first time I asked GP and saw physio - so it's worth letting them know that you really are in a lot of pain, your life is very difficult at the moment, and that it's not true that nothing can be done.
I've also found that friends are really sympathetic, as most of their pregnancies were fairly straightforward, and that having people come over to see me is really nice, especially when they know in advance that I can't get up and need a bit of looking after.
this said, I also feel like I'm getting worse each week, and am quite daunted at the thought of another 10 or more weeks! I am really really hoping for an early baby - not premature or unsafe, but say 38 weeks instead of 40 or 42! the despair is almost (but not quite) as bad as the pain.
your work situation sounds horrible, is there a union or anyone in HR who can stand up for you? and can anyone (friends, family?) help you out at home when dp is not there? people do say that it gets much better very quickly after the birth, so if you can remind yourself that it's a horrible but temporary disability, whereas some people have much worse disabilities that are never going to go away, and they don't have a lovely baby at the end of it, it can feel a little bit better...maybe!