ring these people up
www.pelvicpartnership.org.uk/
you may have to leave a message, but someone helpful will ring you back, and they know all about it, in all its manifestations. They should definitely have tips on how to get your GP to be more useful, and can usually recommend an physio/osteopath in your area that other women have used.
GPs can unfortunately be very unhelpful, and it can also take ages to get an NHS physio appt. Even my GP - youngish woman with children - said 'oh, i had SPD too, but I couldn't let it get to me, i had to come into work'. Well, obviously she didn't have it too badly, because if you have it bad, you just cannot move/walk/bear to sit down or whatever enough to be able to do your job.
Gym ball helps, but if you can get to a physio, or even better, a private osteopath (maybe just for 1 or 2 appointments? it's expensive but it's MUCH better than not going), they can give you some exercises that may relieve some of the pain, and also have a proper feel of your pelvis etc and tell you what's going on there. Rest lots. Don't walk more than you have too, but find out what sort of stretches and exercises you can do, so that you don't get too stiff. Getting down on the floor on all fours and sticking your bum in the air looks silly but helps me a lot. Also gently rolling back/forwards/around and around while sitting on gym ball may help. Lots of pillows in bed around back/between legs. Warm baths? Avoid sitting on sofas (lie on them if you have to). If you sit, try to be very straight-backed, with legs at right angles and feet on floor. But you may well find it impossible to sit in an office job for the remainder of your pregnancy.
Pelvic Partnership again will have better tips than me.
Also - some NHS physios will say 'nothing you can do, just do lots of pelvic floor exercises', and that's not enough, you do need other exercises too (which is why osteopaths seem to be a bit better). Pregnancy yoga, with an instructor who knows a bit about the condition, or aquanatal type classes might be good too?
I agree with poster above about maybe doing mat leave a bit earlier, although I know that 28 weeks sounds horrific, and you shouldn't have to start that early. With my first pregnancy, I was signed off from 28 weeks, but I think at something like 34 or 35 weeks, HR said I HAD to start maternity leave. Can't remember details, was pissed off at the time, but also in so much pain that I couldn't be arsed to argue.
Finally, every case is different, and you may have some pain after the birth too (although hopefully it will resolve fairly quickly), but for me, I was lucky, pain went as soon as baby born, and he was/is SOOO worth it. So much so that I'm pregnant again. Also have SPD again, and cursing myself, but I think this one will be worth it too. And then I'm stopping. No more pregnancies for me!
good luck