I unfortunately have severe SPD and have had it from around 20 weeks (am now 34 weeks). I have been on crutches for the past 7 weeks and have a displaced, overlapping pelvis that will not straighten, dislocating sacroilica joints, a 'wobbly tailbone' and hips that gives way and tries to dislocate on a daily basis (has only happened fully once so far thankfully!). Hijacking - sorry.
Make sure your midwife understands that you are not in "discomfort", you are actually in pain. My midwife openly said to me she does not really know anything about the condition and kept saying "I know you are in discomfort"!
Belts and so forth can help but usually only if you are not displaced in the pelvis, as this can increase pain. However, if you are still straight (physio should be able to tell you this) a tightly fitted bump belt, tubigrip or even your dressing gown belt under the bump can help to hold you together. You will know if you are not straight as the belt will cause pain, not help it.
Also, if you have only a bath or shower over the bath, a bath board may help you to get in and out without seperating your legs. Also make sure you do not lock the door to the bathroom. I once got stuck in the shower and had to be lifted out!
Generally you will find the physiotherapist much more informed than the doctors or midwives. There is only limited help they can provide but sometimes that is enough.
Basically the rule of thumb is do not push yourself, if it hurts, don't do it. Work hard at your pelvic floor exercises as this will help take pressure off the pelvis, also every time you go from sitting to standing or up and down stairs clench your buttocks hard as this will also help. Another good thing is get yourself a birthing ball and do what they call hoola exercises (sided to side, back to front) as this will strengthen your side muscles, again supporting the pelvis. Do not do the squatting, lying or feet up style positions, just sit on it and wiggle. Obviously keep your legs together as much as possible as said before. Also tummy tilt exercises can help.
I hope some of this helps. Sorry it is so long!