Hi Shelley sorry to hear you are feeling so rubbish.
I have PGP and the pain I get is exactly as you describe - it originates in my lower back and spreads down into my bum and legs a bit like sciatica too. I have recently been in so much pain I've been stuck in bed crying my eyes out telling my DP I no longer want to be pregnant!
Unfortunately in my area there is 6 month waiting list to see the NHS physio so I will probably have had the baby before I get an appointment My midwife recommended seeing an osteopath, as they are often better at treating pregnancy related pain than physiotherapists. I googled for one with experience of treating pregnant women, and I had my first appointment today.
I'm fairly impressed by my first experience. She took a detailed medical history from me, and then carried out a very thorough assessment of my posture, spine, pelvis and range of motion. She deduced that as well as having an unstable sacroiliac joint, the bottom of my spine was very fixed and not curving as it should. She then manipulated my joints quite a bit, and carried out some deep tissue massage on my hips. She also did some stuff to my spine which made it click and pop! It was certainly uncomfortable at times but not overly painful. She gave me some advice re. not hoovering, pushing shopping trolleys, sleeping with support pillows etc, and also applied some cool gel to the bottom of my back. The whole session took an hour.
After the appointment I could definitely feel my joints moving more easily, although I am still uncomfortable and she did warn me I may feel worse over the next 24 hours. I'm going back on Monday for another appointment to see if things have improved. The osteopath reckoned 2 or three sessions should be enough to get me functioning again, with a final session to ensure all is well for labour when I get to about 37 weeks.
Hope you find your osteopath experience as helpful as mine - I also wear a support belt and find this lessens the pain although it doesn't take it away completely.